Selective growth of ZnO nanorods on microgap electrodes and their applications in UV sensors
© Humayun et al.; licensee Springer. 2014
Received: 9 October 2013
Accepted: 5 January 2014
Published: 15 January 2014
Selective area growth of ZnO nanorods is accomplished on microgap electrodes (spacing of 6 μm) by using a facile wet chemical etching process. The growth of ZnO nanorods on a selected area of microgap electrode is carried out by hydrothermal synthesis forming nanorod bridge between two electrodes. This is an attractive, genuine, direct, and highly reproducible technique to grow nanowire/nanorod onto the electrodes on selected area. The ZnO nanorods were grown at 90°C on the pre-patterned electrode system without destroying the electrode surface structure interface and geometry. The ZnO nanorods were tested for their application in ultraviolet (UV) sensors. The photocurrent-to-dark (I ph/I d) ratio was 3.11. At an applied voltage of 5 V, the response and recovery time was 72 and 110 s, respectively, and the response reached 2 A/W. The deposited ZnO nanorods exhibited a UV photoresponse that is promising for future cost-effective and low-power electronic UV-sensing applications.
Metal-oxide-semiconductor nanostructures have received considerable attention worldwide because of their excellent physical and chemical properties in the recent past . Among them, zinc oxide (ZnO) nanostructures have attracted significant interest because of their large wide direct bandgap (E g = 3.37 eV)  and high exciton binding energy (60 meV) [2–4]. Ultraviolet (UV) photodetectors are widely used in various commercial  and military applications , such as secure space-to-space communications , pollution monitoring, water sterilization, flame sensing, and early missile plume detection . Moreover, the direct flow of electrons contributes to the maximum photocurrent generation because of the large interfacial surface area . In contrast to GaN, ZnO has a maximum electron saturation velocity; thus, photodetectors equipped with ZnO can perform at a maximum operation speed . Different types of photosensors, such as p-n junction, metal–semiconductor-metal, and Schottky diodes, have been fabricated. However, metal–semiconductor-metal photosensors are becoming popular because of their simple structure . The sensor photoconductivity of ZnO depends on the growth condition, the surface morphology, and crystal quality .
The synthesis of ZnO nanostructures has been reported; however, the area-selective deposition of ZnO nanostructures or their integration into complex architectures (microgap electrode) is rarely reported [13–24]. In this manuscript, we report the deposition of ZnO nanorods on a selective area of microgap electrodes through simple low-cost, highly reproducible hydrothermal technique, and their applications in UV sensors were investigated.
Materials and method
Results and discussion
Figure 1a shows the schematic view of entire experimental process. Figure 1b shows the butterfly topology zero-gap chrome mask. Figure 2a,b,c shows high- and low-magnification SEM micrographs of the deposited ZnO nanorods. The SEM showed the morphological features of the ZnO nanorods deposited on a selected area of microgap electrodes. The seeded area was completely covered with ZnO nanorods which indicates selective growth on the area of microgap electrodes. It is noteworthy to mention that the as-grown ZnO nanorods were interconnected to each other as noticeably seen by the SEM observations [29–31]. Such interconnected network facilitates electron transport along the nanorod/nanowire axis [32, 33].
where O2 is the oxygen molecule, e- is the free electron and the photogenerated electron in the conduction band, is the adsorbed oxygen, hv is the photon energy of the UV light, and h+ is the photogenerated hole in the valence band. After the UV light is switched on, the number of oxygen molecules on the ZnO nanorod surface rapidly reaches the maximum value in response to the ultraviolet light . When the ultraviolet light is switched off, the oxygen molecules are reabsorbed on the ZnO nanorod surface. Thus, the sensor reverts to its initial mode .
An important parameter used to evaluate the suitability of the sensor for UV-sensing applications is spectral responsivity as a function of different wavelengths. This parameter yields the internal photoconductive gain.
In summary, the ZnO nanorods were selectively grown on pre-patterned seeded substrates at low temperature (90°C) by hydrothermal method. Conventional lithography followed by simple wet etching process was used to define microgap electrodes with approximate spacing of 6 μm on seeded substrates. The ZnO nanorod microgap electrodes were investigated in dark and UV environments and showed noticeable changes with UV light exposure. The sensor gain was 3.11. The response time was less than 72 s. The recovery time was 110 s. The responsivity was 2 A/W. These fascinating results propose that the selective area growth of the ZnO nanorods exhibits a UV photoresponse that is promising for future cost-effective and low-power electronic UV-sensor applications.
QH is a PhD Student at the Institute of Nano Electronic Engineering University Malaysia Perlis. MK is a Post Doctorate Fellow at the Institute of Nano Electronic Engineering University Malaysia Perlis. UH is a Professor and Director of the Institute of Nano Electronic Engineering University Malaysia Perlis. AQ is an Assistant Professor at the Center of Excellence in Nanotechnology and Chemistry Department of King Fahd University of Petroleum and Minerals, Saudi Arabia.
The authors acknowledge the financial support from the Ministry of Higher Education (MOHE). The authors would also like to thank the technical staff of the Institute of Nano Electronic Engineering and School of Microelectronic Engineering, Universiti Malaysia Perlis for their kind support in the smooth performance of the research.
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