Fabrication and spectroscopic investigation of sandwich-like ZnO:rGO:ZnO:rGO:ZnO structure by layer-by-layer approach

Research highlights

• ZnO/rGO/ZnO/rGO/ZnO and rGO/ZnO/rGO/ZnO/rGO sandwich structures were fabricated.

• The ordering of sandwich layer's were altered the microstructural parameters.

• Bandgap narrowing of the ZnO upon the order of sandwich layer were observed.

• The photoluminescence of sandwich structures were investigated.

Abstract

Transparent conducting materials (TCMs) are the heart of modern optoelectronic industries and the properties of TCMs could be improved by the introduction of 2D carbon materials. In this report, the influence of order layering on microstructural, transparency and emission characteristics of ZnO:rGO:ZnO:rGO:ZnO and rGO:ZnO:rGO:ZnO:rGO sandwich structures has been investigated. The layer-by-layer approach has been adopted for the fabrication of sandwich structured materials ZnO:rGO:ZnO:rGO:ZnO and rGO:ZnO:rGO:ZnO:rGO through the spin coating technique. The sandwich structures of ZnO and rGO exhibited hexagonal wurtzite structure of ZnO without any impurities were identified through XRD. The ordering of layer's influenced the microstructural parameters and were significantly altered. The spherical nature of the particles and the formation of the sandwich structures were confirmed by using SEM micrograph. The reduction in an optical transparency and narrowing bandgap of the ZnO upon the order of layering were identified through transmission spectra. The lower energy shift of near band edge (NBE) emission and reduction in the emission intensity with respect to pure ZnO nanostructures was observed. The present work provides a simple layer-by-layer approach to fabricating sandwich structures and improving the optical properties which have potential applications in various optoelectronic devices.

Introduction

Transparent conducting materials (TCMs) have attracted enormous interest in recent years across the globe due to their potential use in optoelectronic devices [1], photovoltaics [2], sensors [3], and novel display devices [4], [5]. The prime requirement for the TCMs is more than 90% of optical transparency in the visible regime and very low resistivity of 10^-4 Ω.cm [6]. Tin-doped indium oxide (ITO) materials have been fulfilled with these prime requirements and it’s been widely used in the optoelectronic industries. However, due to its brittle in nature, high production cost and future unavailability, there is an extensive demand for the search of novel materials and fabrication technologies [7], [8]. The doped zinc oxide (ZnO) materials are widely used as TCMs and to improve the conductivity of ZnO, various materials have been introduced which include, doping of group III elements (Al and Ga) [9], [10] silver:ZnO nanocomposites [11] and introduction of two-dimensional (2D) materials [12], [13].

Among various two-dimensional (2D) structured materials, graphene has drawn significant consideration because of its extraordinary electrical conductivity (2x10⁵ cm²V^-1s⁻¹) [14], thermal (5000 W.m^-1K⁻¹) [15], mechanical properties [16] and also exhibits high optical transmittance in the visible regime [17]. Wu et al [18] have synthesized sandwich-like graphene/ZnO (5 nm diameter) nanocomposite materials and identified that the stacking of graphene sheets could be well controlled through decoration of densely packed ZnO nanostructured particles. Li et al [19] have fabricated 3D sandwich-structured ZnO/rGO/ZnO nanocomposites using rapid thermal reduction routes. The sandwich structured materials exhibited maximum capacitance (275F g⁻¹) with anomalous rate capability and cycling stability in comparison with their parent materials. Teh et al [20] fabricated the rGO-hybridized ZnO sandwich thin films using facile electrodeposition route through layer-by-layer technique. Results revealed that the ordering of layer is highly influenced in the charge transfer properties which ultimately improves the efficiency of photocatalytic electrochemical water splitting. Boukhoubza et al [21] have prepared the sandwich structure of GO/ZnO nanorods/GO and evaluated their emission characteristics. The results showed that the decoration of GO layers leads quenching of PL emission intensity attributed to the charge transfer process. To date, a limited studies have been performed for the fabrication and understanding of ZnO/rGO sandwich structures and yet to be explored in details.

Considering the importance of ZnO/rGO sandwich structures in various energy and opto-electronics industries, in this report a simple layer-by-layer approach using spin coating technique has been adopted to prepare ZnO:rGO:ZnO:rGO:ZnO and rGO:ZnO:rGO:ZnO:rGO sandwich structures. The effect of layer ordering of sandwich structures on the microstructural and optical emission characteristics was investigated. Furthermore, the optical transparency of sandwich structures is analysed in a detailed manner.