Elsevier

Journal of Alloys and Compounds

Volume 777, 10 March 2019, Pages 145-151
Journal of Alloys and Compounds

Synthesis of Pd:ZnO nanofibers and their optical characterization dependent on modified morphological properties

https://doi.org/10.1016/j.jallcom.2018.10.325Get rights and content

Highlights

  • A Facile and efficient growth method to fabricate the Pd:ZnO nanofibers.

  • The surface roughness values decreased due to Pd doping.

  • The Eg of the samples increase with Pd doping due to the Burstein-Moss shift.

  • The refractive index distribution was analyzed by using the Wemple-Didomenico model.

  • The conductivity values increased with increasing energy.

Abstract

In this study, Pd:ZnO thin films were grown on microscope glass by the sol-gel spin coating method to investigate and improve interaction between surface properties and optical properties. Morphology of the obtained nano-structured thin films was examined using Atomic Force Microscopy (AFM). AFM images showed that the samples were formed in the nanogranule and fiber form. The size and shapes of the fibers varied depending on the Pd concentration. The effect of Pd doping on the optical properties of ZnO films was investigated by spectrophotometric measurements. Optical constants (refractive index, n, and absorption index, k) of the samples were obtained in the wavelength range of 300–800 nm. Dispersion parameters were determined and discussed according to the single oscillator model. As a result of the investigations, it was determined that Pd doping has significant effects on the morphological and optical properties of ZnO films and the produced samples can be used in transparent conductive electrode applications, optoelectronic devices, and sensor production.

Introduction

The production of semiconductor materials has opened the door to a modern world and technological developments. Semiconductors are the basic materials of today's advanced electronics industry and are used in the production of many devices. Nowadays, intensive studies on semiconductor materials are continuing since primary energy resources are starting to run out over time, and due to the emergence of different energy problems and the search for alternative energy sources.

Because of their superior properties, the importance of group II-VI semiconductor compounds is increasing with each passing day due to the advantages of their use in advanced technology applications [1]. As a member of this group, zinc oxide (ZnO) is an n-type semiconductor with a broadband gap at room temperature [2,3]. The optical band gap of the ZnO compound is 3.2–3.37 eV [4,5], and it has high exciton binding energy (60 meV) [6,7].

Nowadays, because of their broad and direct band gap and ability to react strongly to UV [8], doped and undoped ZnO thin films are widely used as optoelectronic circuit elements [9], surface acoustic wave devices [10], photovoltaic solar cells [11,12], gas sensors [13] and photodiodes [14].

The physical properties of ZnO thin films according to their usage areas can be changed considerably by means of appropriate doping, and intensive research is being carried out to bring these properties to the highest level [15]. Therefore, Pd doping is often used to modify the surface reactions of ZnO and improve its physical and perception properties. For example, Wang et al. [16] reported that Pd2+-doped ZnO nanotetrapods were prepared and studied for the humidity detection application. Kashif et al. [17] revealed that Pd-doped ZnO nanorods were synthesized using a sol-gel spin coating technique for hydrogen sensing applications. They exhibited homogeneous surface morphology, c-axis orientation, and excellent crystalline properties. Pure and Pd-doped ZnO nanofibers were synthesized by an ES method, and the CO sensing properties were investigated by Wei et al. [18]. Additionally, Wang et al. [19] indicated that Pd2+-doped ZnO nanotetrapods modified quartz crystal sensor were prepared and studied for the detection of ammonia. However, the number of studies dependent on the modified morphological properties of Pd-doped ZnO thin films prepared by using the sol-gel method is still limited.

Semiconductor films, which play a significant role in scientific and industrial studies, are currently produced with many production methods such as the sol-gel spin coating method [20], spray pyrolysis [21], pulsed laser deposition [22], chemical vapor deposition (CVD) [23], and RF sputtering [24]. Among these methods, the sol-gel spin coating method is widely used because of its advantages, such as low cost compared to other methods, being a simple coating method, applicability to wide surfaces and the easy adjustment of doping concentration [17,25].

In this research article, Pd was added at different rates to the solutions prepared at the same concentrations. The effects of the surface morphology of ZnO nanostructured materials on the optical properties were investigated depending on the changing doping concentration. It is foreseen that this study will shed light on the steps to be taken since studies on the production of Pd-doped ZnO nanofibers with low costs and their optical characterization dependent on modified morphological properties are not frequently encountered in the literature.

Section snippets

Experimental setup

In this study, undoped and Pd-doped ZnO samples were produced by the sol-gel method. Coating of gels obtained by the sol-gel method on microscopy glass was performed by the spin-coating method.

Morphological characterization of the prepared nanostructured Pd:ZnO thin films

The AFM images of undoped and Pd-doped ZnO samples, taken at different magnifications (40μx40μ and 5μx5μ), are presented in Fig. 1(e and f). From the obtained AFM images, it is seen that the surface morphology of the samples is formed by the growth of nanogranules in the fiber form. These fibers are dense, irregular, and homogeneously distributed across the surface of the films. The size and shapes of the fibers vary depending on the Pd concentration. This change in the fiber size is attributed

Conclusions

Undoped and Pd-doped ZnO thin films were grown on glass base plates with the sol-gel spin coating technique. The effect of Pd concentration and microstructural properties on optical properties was investigated. Surface morphology studies carried out with AFM showed that with Pd doping the homogeneity and surface roughness of ZnO thin films in nanogranule-fiber structure tend to decrease. Optical spectroscopy spectra show that the optical band gaps of the samples increase with Pd doping that is

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