Materials Science in Semiconductor Processing 106 (2020) 104763 Available online 7 October 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved. Controlled growth of 1D-ZnO nanotubes using one-step hot plate technique for CZTS heterojunction solar cells S. Varadharajaperumal a, d, * , Devarajan Alagarasan b , R. Ganesan b , M.N. Satyanarayan d , Gopalkrishna Hegde c, ** a Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, 560012, India b Department of Physics, Indian Institute of Science, Bengaluru, 560012, India c BioSystems Science and Engineering, Indian Institute of Science, Bengaluru, 560012, India d Optoelectronics Laboratory, Department of Physics, National Institute of Technology, Karnataka, Surathkal, Mangalore, 575 025, India A R T I C L E INFO Keywords: Semiconductors ZnO Crystal structure XPS Hot plate technique ABSTRACT Present work reports a simple, rapid, one-step hot plate technique for systematic growth transformation of highly oriented ZnO nanorods (ZNRs) into ZnO nanotubes (ZNTs). The controlled growth of ZnO nanostructures (nanorods and nanotubes) was achieved at low temperature (90 � C) in a short time (1hr) in a sealed weighing bottle (100 ml). It is observed that as the Zinc precursor concentration increases, a vertically grown ZnR morphology evolves into ZNT. The crystal structure of as-grown ZnO nanostructures, surface morphology, phase, and optical energy gap were respectively characterized by XRD, FESEM, Raman, XPS, CL and UV–Vis spec- troscopy. Grown nanostructures are further explored for their application in CZTS based heterojunction photovoltaics. 1. Introduction One-dimensional (1D) Zinc oxide (ZnO) nanostructures are regarded as one of the most fascinating wide bandgap oxide semiconductors owing to its optical transparency in the visible region augments the excited charge carrier separation and transportation in various appli- cations [1–5]. In recent years, several research efforts have been devoted to growing hollow 1D-ZNTs, due to its unique properties such as large surface area, high porosity, light trapping and high carrier mobility which attract their applications in photo catalysts, gas sensor, biosensor, bio-photonics and solar cells [6–10]. Considering the broad range ap- plications of ZNTs, the growth process generally demand cost effective, rapid, large scale and modest techniques. So far, highly oriented ZNTs (over various substrates) were successfully synthesized from template assisted, hot plate assisted electro deposition, hydrothermal and solution processed techniques [7,11–13]. Among all the aforementioned methods, the simple hydrothermal technique offers an advantage to yield highly scalable, reproducible and most controllable production of ZNTs at low synthesis temperature (<350 � C) [12]. In general, hydrothermally grown ZNTs were synthesized by two- step processes (excluding seed layer deposition). The frst step is to prepare ZNRs by controlling the experimental parameters like reactant concentration and volume, reaction temperature and time, substrate positioning, pH and surfactant. The second step is to etch the grown ZNRs using electrolyte solutions, which mainly depends on solution concentration and volume, etching time and temperature [7,14]. Due to this complex two-step process, there is an urgent requirement to fnd a better alternative to replace it with simple, rapid and low-cost direct synthesis technique to grow ZNTs in large scale with comparable or better material, optical and electronic properties. To date, only a few reports are available on the direct growth process of ZnO tubular structures via conventional hot air oven method by keeping either long reaction time or subsequently cooling down the reaction temperature from high to low or post-deposition aging route [4,15]. In recent years there has been a lot of interest in fabricating the superstrate structure (a reverse process which is used to prepare con- ventional substrate cell) based CZTS thin-flm solar cells combining with 1D nanostructures. In addition, the superstrate structure has been considered as one of the most suitable approaches to study the advan- tages of introducing 1D nanostructures in heterojunction as compared to * Corresponding author. Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, 560012, India. ** Corresponding author. E-mail addresses: varadhu@iisc.ac.in (S. Varadharajaperumal), gopalkrishna@iisc.ac.in (G. Hegde). Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp https://doi.org/10.1016/j.mssp.2019.104763 Received 31 May 2019; Received in revised form 12 September 2019; Accepted 2 October 2019