Electronic excitations induced modifications of structural and optical properties of ZnO–porous silicon nanocomposites
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 (Sn), or electronic excitation and ionization of target atoms by inelastic collision i.e. electronic energy loss (Se). The second process is predominant in case of SHI irradiation. It supplies huge excitations (Se) as compare to that produced by elastic collisions (Sn), 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.
Section snippets
Experimental
The porous silicon was formed on boron-doped silicon [1 0 0] with resistivity ∼0.8–1.2 Ω-cm using electrochemical anodization. The aluminum films were evaporated on the back of the silicon for a good ohmic contact. The anodization was carried out in the HF-ethanol solution (HF-48%: Ethnol-98% = 1:1) for 20 min at a constant current density of 30 mA/cm2 at room temperature. After the anodization PS samples were rinsed in ethanol, then dried by blowing and finally stored in an atmospheric environment at
GAXRD measurements
Fig. 1 shows the GAXRD pattern of the pristine and irradiated films of ZnO/PS nanocomposites. It is observed from the figure that the film exhibits a dominant peak at 2θ = 36.34 corresponding to the (1 0 1) plane of ZnO. Other peaks corresponding to (0 0 2), (1 0 0) and (1 0 2) (matched with ICDD data base with JCPDS-79-0206). The XRD pattern of ZnO shows the polycrystalline nature with wurtzite structure. The lattice constants of a = b = 3.25 Å, and c = 5.20 Å are calculated from the given GAXRD pattern on
Conclusions
The good quality ZnO nanocrystallites with wurtzite structure were deposited into the pores of porous silicon (PS). The XRD and PL results show that encapsulated ZnO crystallite in the pores of PS exhibit stronger structural modifications as compare to ZnO films by SHI irradiation. SHI induced oxygen diffusion from ZnO lattice and oxidation of PS surface is invoked to understand the mechanism of structural modifications. The observed modification in structural and optical properties ZnO–PS
Acknowledgements
Authors are thankful to Pelletron group of IUAC for providing the stable beam for SHI irradiation experiment under the project code UFUP-37320. One of the authors (RGS) is thankful to UGC for providing senior research fellowship (SRF) and Dr. D.K. Avasthi for providing the moral support. Another author (VA) is acknowledged the financial support from CONACyT (57631). DST is also acknowledged for providing XRD and AFM facilities to IUAC through IRPHA project.
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