Elsevier

Materials Letters

Volume 264, 1 April 2020, 127370
Materials Letters

Microwave irradiation assisted rapid growth of ZnO nanorods over metal coated/electrically conducting substrate

https://doi.org/10.1016/j.matlet.2020.127370Get rights and content

Highlights

  • Growth of ZnO nanorod on a conducting substrate by microwave irradiation.

  • Deposition method is very easy and cost effective.

  • c-Axis oriented growth is achieved.

  • Growth mechanism is discussed.

  • Growth method is scalable for variety of substrate.

Abstract

In-spite of intense research in the growth of ZnO over the years, the direct crystallization of ZnO on a metal coated/electrically conducting substrate by a solution growth process has not been carried out. We report one pot, low temperature (≤100 °C) and rapid growth (5 min) of ZnO nanorods over an electrically conducting substrate by using microwave irradiation assisted thin film deposition in a liquid medium without the use of any seed layer. High density (2 × 109/cm2), c-axis oriented, uniform and continuous ZnO nanorod films are grown over Cr/Si [chromium coated Si(1 0 0)] and ITO/glass (indium tin oxide coated glass) substrates whereas aluminium (Al) and gold (Au) coated Si yield non uniform film with mixed texture. Microwave irradiation for 5 min ensures strongly oriented growth of ZnO over Cr/Si and ITO/glass and mixed texture for semiconducting substrates (Si/Ge). The initial nucleation of ZnO nanorods may be due to the formation of thin amorphous oxide (Cr2O3) layer over Cr film, which subsequently changes to hydroxides (CrOH2, CrOH3) in contact with water.

Introduction

ZnO, being an environmental friendly material has stimulated persistent research over the years due to the easy and straightforward fabrication process along with numerous applications as reviewed previously [1], [2]. Several thin film deposition methods (gas phase or wet synthesis) have been conceived to obtain fancy ZnO nanostructures over a substrate [3], [4]. In a thin film deposition process, the electrical conductivity measurement is carried out by depositing a metal layer on the top surface of the film (if the film is uniform), because the substrates, such as highly doped Si or Ge, may themselves serve as the contact at the “other end”. But if the film is not of uniform thickness, and comprises nanorods grown vertically on the substrate, then it becomes tedious to get metal contacts at both ends. It then becomes desirable to grow such nanorods on a uniformly deposited metal layer on a substrate, and establish electrical contact on the “top” of the nanorod by lithography. Metal catalysts/metal films are used previously for the growth of ZnO nanorods over a substrate by a high temperature vapor liquid solid mechanism [5], [6], [7], However, the metal catalysts often generate unintentional defects by diffusion and remain on the top of the nanorods/nanowires after the growth and that hinders further device integration. Large scale growth of ZnO nanorods over a metal coated substrate by wet chemical synthesis and the growth mechanism has not been considered in great detail. Microwave irradiation (M.I.) assisted thin film deposition technology in a liquid medium has been used successfully to obtain ZnO coatings over semiconducting/dielectric substrates [8], [9], [10]. Here, we have taken an initiative to deposit ZnO over a metal coated Si substrate quickly by M.I assisted method to make this method more versatile and useful. Metals usually reflect electromagnetic radiation and therefore it would also be interesting to investigate the effect of microwave over the crystallization and growth of ZnO on an electrically conductive surface placed in the precursor solution.

We demonstrate the large scale growth of ZnO nanorods on a metal coated Si (1 0 0) in 5 min without using ZnO seed layer. The growth is carried out by M.I. in liquid medium, and the growth temperature is expected to be relatively low (≤100 °C).

Section snippets

Experimental details

In a typical procedure, the ethanol solution of zinc acetylacetonate [(C10H14O4. Zn), 5 mmol (1.31 g)] was mixed with the aqueous solution of cetyltrimethylammonium bromide (CTAB) (∼0.728 g in 40 ml distilled water) in a 250 ml round-bottomed flask and stirred for fifteen minutes. A substrate (Cr/Si, Al/Si, Au/Si) was dipped into the above solution mixture (Fig. S1), and microwave is irradiated (800 W) for 5 min (supplementary information). The substrate was removed after M.I., cleaned with

Results and discussion

Fig. 1 illustrates the structure/texture and microstructure of the ZnO nanostructured thin films over a Cr/Si substrate. The XRD pattern shows sharp and high intensity (0 0 2) peak (Fig. 1a). Fig. 1b shows the corresponding (1 0 0) and (0 0 2) X-ray pole figure which affirms the strongly oriented growth along (0 0 2) direction. The coating comprising of the nanorods is uniform, continuous, and that covers over a large area of 625 mm2 (Fig. 1c). The plan view (Fig. 1c) shows the top faces of the

Conclusions

Microwave irradiation assisted thin film deposition technology is used efficiently to deposit ZnO coatings over a conducting substrate. The texture of ZnO coatings strongly depends on the substrate surface. The as obtained coatings do not need any post deposition processing. This method may be extended to deposit ZnO on other metal coated substrates for the wide range of applications in nanoelectronics.

CRediT authorship contribution statement

Sanjaya Brahma: Conceptualization, Investigation, Methodology, Writing - review & editing, Data curation, Writing - original draft. S.A. Shivashankar: Conceptualization, Visualization, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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