Abstract
Zinc oxide (ZnO) has many useful properties for electronics and optoelectronics including wide band gap, large exciton binding energy, low-cost, ease of processing and availability [1]. This has led to increased interest and development of thin film electronics, transparent conductors, solar cells, light-emitting diodes, lasers, photodetectors and various sensors based on ZnO materials [2, 3]. The high surface areas and tunable properties of ZnO nanostructures make them particularly suitable for applications such as sensing and photonic devices [4, 5].
In this work, we present results on nanostructured ZnO thin film photodetectors fabricated using nanoparticle inks (nanoinks) obtained via planetary ball milling (PBM) of bulk powders. PBM [6] is an emerging solution-based nanofabrication approach that can quickly produce nanoink suspensions at low-cost by nanoscale grinding, without complex processing and suitable for thin film coating of various materials on different substrates [7]. The thin film photodetector devices were fabricated by depositing PBM ZnO nanoink onto flat insulating glass substrates followed by contact formation as shown schematically in Fig. 1a: PBM was performed using ZnO powder in ethylene glycol (EG) or deionized (DI) water solvent (a.k.a. colloidal grinding) with zirconia grinding beads. The grinding speed and time were varied between 200 and 1000 rpm and 10 min. and 60 min., respectively. A few μL of the resulting ZnO nanoink was used to coat the substrate surface and dried at ~ 100 °C. Lastly, two electrical contacts to the resulting films were made using silver paint and copper tape. Analysis of ZnO films after deposition showed they consist of nanostructured particles with sizes reaching below 100 nm, as displayed in the scanning electron microscopy (SEM) and atomic force microscopy (AFM) images in Fig. 1b, depending on grinding conditions (speed, time).
The optical properties of the ZnO thin films were evaluated via photoluminescence measurements (Fig. 1c), which, in addition to interband transitions in the UV, displayed longer wavelength emission peaks due to surface and bulk defect states. Such deep level states are dependent on grinding parameters and thus allow the accessible spectrum for the nanostructured ZnO films to be extended into the visible region in a tunable manner. Two-terminal photoconductance data of the ZnO PBM nanoink thin film devices were obtained using a probe station and precision source-measure unit, with and without illumination, under ambient atmosphere and at room temperature. Fig. 1d shows current vs. voltage curves obtained for a typical photodetector device, which display current increasing proportional to incident light intensity. This behavior can be explained by electron-hole pair creation and desorption of surface oxygen species (leading to vacancies that act as donors) upon photon absorption, which leads to an increase in conductance. This is consistent with previous studies where ZnO thin films have been used for photoconductive sensor applications, validating PBM nanoink as a suitable synthesis technique for the active material in photodetectors.
Compared to standard ZnO thin films, the PBM nanoink method allows both particle dimensions and surface states to be tailored and optimized for different photodetector applications in a straightforward manner by adjusting grinding conditions. In particular, both UV and visible light detection can be tuned via the solution-based ZnO nanoink approach presented without additional material/chemical processing. Such PBM nanoinks thus offer the potential of realizing low-cost photodetectors and multifunctional thin film coatings for applications in optoelectronics, imaging, environmental monitoring and communications.
References
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Figure 1