Electronic excitations induced modifications of structural and optical properties of ZnO–porous silicon nanocomposites

Abstract

The influence of swift heavy ion (SHI) irradiation on structural and photoluminescence (PL) properties of ZnO nanocrystallites deposited into porous silicon (PS) templates by the sol–gel process was studied. The ZnO/PS nanocomposites were irradiated using 120 MeV Au ions at different fluences varying from 1 × 10¹² to 1 × 10¹³ ions/cm². The intensity of the X-ray diffraction peaks is suppressed at the high fluence, without evolution of any new peak. The PL emission from PS around 700 nm is found to decrease with increase in ion fluence, while the PL emission from deep level defects of ZnO nanocrystallites is increased with ion fluence. At the highest fluence, the observation of drastic increase in PL emission due to donor/acceptor defects in the region 400–600 nm and suppressions of XRD peaks could be attributed to the defects induced structural modifications of ZnO nanocrystallites.

Introduction

Template-synthesis strategies are becoming more and more popular for the preparation of novel nanocomposite materials. In last few years, there were a lot of research efforts have been devoted to understand the properties of ZnO based nanostructures for advanced opto-electronic applications [1]. In particular, ZnO/porous silicon (PS) nanocomposites have potential applications as inorganic solid state white light sources (ISSWLS) [2], [3], [4], [5]. SSWLS is being realized by the combination of orange-red emission from the PS [6] with the UV-blue and green emission from the excitons and deep level defects from the ZnO nanocrystallites. Other important application of these nanocomposites materials has received extensive attention because of their potential applications in various fields. They will play a crucial role in improving future optoelectronic devices [7], [8], data storage [9], [10], bio-chemical and chemical sensors [11], [12] along with efficient field emission properties [13]. But till now this phenomenon is under discussion due to very limited understanding about the process of carrier injection from PS to ZnO crystallites. The interaction between the enormous surface to volume ratio of nanoparticles and surrounding medium, strongly affects the emission spectrum [14]. These nanocomposites offer an effective means to engineer its optoelectronic properties and can be easily integrated in the existing silicon based technology for various applications in the field of light emitting technology. On the other hand, the sponge like open structure and large specific surface area make PS a convenient material [15] for accommodating ZnO crystallites into its pores. The nanocomposites can be synthesized by various deposition techniques such as sol–gel [16], chemical vapor deposition [17], pulse laser deposition [18], RF sputtering [19], RF magnetron sputtering [20] and vapor–liquid–solid mechanism [21]. But, for the present study sol–gel process is used for being very simple, controlled and cost effective.

The defects in ZnO can be altered by various treatments like thermal annealing in controlled environment or by ion beam irradiations. It is known that the PL emission of PS can be stabilized by swift heavy ion (SHI) irradiation [22] and thin films of ZnO can not be amorphized by ion irradiations [23], [24]. SHI irradiation is a unique technique to modify structural, optical, optoelectronic and transport properties of materials [25], [26]. When SHI penetrate a solid, it slows via two processes of direct transfer of energy to target atoms through elastic collisions i.e. nuclear energy loss (S_n), or electronic excitation and ionization of target atoms by inelastic collision i.e. electronic energy loss (S_e). The second process is predominant in case of SHI irradiation. It supplies huge excitations (S_e) as compare to that produced by elastic collisions (S_n), if the thickness of target is much smaller as compare to the range (∼9 μm) of the projectile ions. SHI modifies the properties by two processes of; (i) controlling the type and density of defects [27], and (ii) by creating the stress and strain in the structure [28]. The aim of the present work is to study the effects of SHI irradiation on the sol–gel grown nanocomposites for stabilizing the PL emission and structural modifications of ZnO crystallites in the pores of PS.