Photo-Response of Functionalized Self-Assembled Graphene Oxide on Zinc Oxide Heterostructure to UV Illumination
© Fouda et al. 2016
Received: 15 October 2015
Accepted: 27 December 2015
Published: 12 January 2016
Convective assembly technique which is a simple and scalable method was used for coating uniform graphene oxide (GO) nanosheets on zinc oxide (ZnO) thin films. Upon UV irradiation, an enhancement in the on-off ratio was observed after functionalizing ZnO films by GO nanosheets. The calculations of on-off ratio, the device responsivity, and the external quantum efficiency were investigated and implied that the GO layer provides a stable pathway for electron transport. Structural investigations of the assembled GO and the heterostructure of GO on ZnO were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The covered GO layer has a wide continuous area, with wrinkles and folds at the edges. In addition, the phonon lattice vibrations were investigated by Raman analysis. For GO and the heterostructure, a little change in the ratio between the D-band and G-band was found which means that no additional defects were formed within the heterostructure.
Much attention has been attracted to the coupling of graphene oxide (GO) and graphene (GR) with some semiconductors, which makes a proper enhancement in the charge transport, photocatalytic activity, and thermal conductivity [1–4]. In particular, GO/ZnO heterostructure is desirable for the inverted structure of hybrid solar cells , transparent electrode in optoelectronic devices , photocatalytic active devices , and sensors . Zinc oxide (ZnO) has a large exciton binding energy of 59 meV, wide band gap of 3.37 eV at room temperature, piezoelectricity, catalytic activity, low cost in production, and bio-compatibility and is non-toxic (environmental friendly) and chemically stable [9, 10]. It has a wide range of applications, like transparent electrodes, gas sensors, dilute magnetic semiconductors (DMS), window layer for solar cells, active channel layer of transparent thin film transistor (TTFT), photocatalysts, surface acoustic wave devices, microsensors, and photodetectors [11, 12].
Graphene, a flat monolayer of two-dimensional (2D) honeycomb carbon atoms, has a wide range of applications due to its superior structural and electronic properties [13–15]. It can be synthesized by several methods, including micromechanical exfoliation , thermal expansion , chemical vapor deposition , and reduction from GO [19, 20]. Recently, there has been much progress in the self-assembly of nano-colloidal particles for photonics, sensors, supercapacitors, electronics, and other applications. Self-assembly technique provides a facile, rapid, inexpensive, scalable, controllable, and good way to deposit nano- and micrometer-sized particles. Controlling the interactions among particles and particle kinetics is required for device fabrication using the self-assembly method [21–23]. The colloidal composition, concentration, and system setup were considered while performing the experiment (more details about the experiment procedure can be found in the “Experimental” section).
Some studies have primarily focused on ZnO-based photodetectors [24–27]. ZnO-based nanostructured photodetectors exhibited a relatively long response time . However, building an electric field within a heterostructure junction is one of the strategies to separate and transport the photo-carriers . In the open literatures, there are some attempts to introduce GR-ZnO nano-composites, ZnO nanowires, GR arrays/films [30, 31], GR-ZnO nanorods [32, 33], and GR wrapped to hollow ZnO spheres . Moreover, resistance switching of GR to ZnO as a resistive random access memory was reported . However, the reports on the assembly of GO on ZnO films and the application of GO/ZnO heterostructure in UV sensing are still quite rare. Here, a low-cost, facile, and scalable technique was used to cover ZnO films by GO nanosheets. We used ZnO thin films as a template for GO to improve the separation efficiency of photo-generated electron hole pairs upon UV irradiation. This hybrid heterostructure exhibited a repeated fast and uniform response to UV illumination because of the high-transport properties of carbon nanostructures.
Structural characterizations were performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Burker-D8 diffractometer with Cukα radiation was used for XRD measurements. The surface morphology was investigated using FE-SEM (Helios 400). The nano-scale structures were monitored using transmission electron microscopy (model JEM 1230, JEOL, Japan). The characterizations were extended to the lattice vibration modes by micro-Raman spectroscopy measurements at room temperature (model Renishaw System 2000) with Ar+ laser at wavelength of 514 nm and power of 3 mW. To evaluate the photoconductivity of the samples, a patterned mask was used to deposit 20-nm-thick Al electrodes (4 mm wide) by thermal evaporation technique as shown in Fig. 1. I-V characteristics were measured at room temperature by a Keithley electrometer (model 6517B). A UV source (254 nm) with power density of 250 mW cm−2 was used to irradiate the samples.
Representative XRD patterns of graphite, ZnO films, and GO on ZnO films are shown in Fig. 2d. For ZnO films on an a-plane sapphire substrate, a well-oriented (0002) ZnO peak can be observed beside the reflexes of the substrate. In the inset of Fig. 2d, the symmetric nature, sharpness of the (0002) peak with full width at half maximum (FWHM) of 0.087°, and the absence of reflections from other planes confirm a good c-axis orientation perpendicular to the (11–20) plane of the sapphire substrate. The distinct sharp (002) peak of graphite was observed at 2θ of 26.55°. After exfoliation, the (002) peak is shifted to a lower angle for GO nanosheets on ZnO films which is related to the increase in the inter-planar spacing beside the reflections of ZnO films and the substrate.
Surface functionalization of ZnO films by the GO layer was conducted by self-assembly technique. ZnO films act as a good template for the deposited GO layer because of its smoothness. It is worth noting that we emphasized the enhancement in the UV photo-response performance for GO/ZnO heterostructure with respect to ZnO films. Since GO creates two-dimensional electronic-conducting channels for the photo-generated carriers, separation and transport of photo-generated electron hole pairs and reducing the recombination improve the on-off ratio.
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