Investigations on LSPR effect of Cu/Al nanostructures on ZnO nanorods towards photodetector applications

Highlights

• Synthesis of Cu and Al coated ZnO NRs for photoluminescence enhancement.

• Cu–ZnO NRs show better surface plasmon resonance effect compared to Al–ZnO NRs.

• PL emission enhancement factor of ~7 was observed in Cu–ZnO NRs and ~2 was observed in Al–ZnO NRs.

• Earth abundant metal coated ZnO NRs show comparable photoluminescence over noble metals coted ZnO nanostructures.

• Cu/Al coated ZnO shows increased UV photodetection in comparison to pristine ZnO NRs.

Abstract

In this paper, we have investigated the room temperature photoluminescence spectra of Cu/Al nanoparticles coated ZnO nanorods for optoelectronic applications. Pulsed laser deposition technique was used to cote the metal nanoparticles on ZnO nanorods. We have done a detailed investigation on photoluminescence spectra of Cu/Al coated ZnO nanorods where we have observed significant enhancement of near band edge emission and remarkable suppression of deep level emission. Such enhancement of near band edge emission and suppression of deep level emission might be due to the arise of several physical consequences, such as exciton–plasmon coupling, hot carrier formation, defect passivation and enhanced electric field at metal (Cu/Al)-semiconductor (ZnO) junction. We have observed ~7.6 times and ~2 times near band edge emission enhancements in Cu and Al coated ZnO nanorods, respectively. Further, a planar type photodetector device was fabricated by using pure Cu as a planar electrode on top of the metal coated ZnO nanorods. We have studied the value of currents in Cu/Al coated ZnO nanorods compared to the pristine ZnO nanorods at dark and UV illumination conditions. We have observed that the current value of Cu coated ZnO nanorods increases nearly 4 orders higher in magnitude in comparison to the pristine ZnO nanorods under UV illumination. However, the value of current in Al coated ZnO nanorods increases little less i.e. ~3 orders higher in magnitude in comparison to the pristine ZnO under UV illumination condition. Hence, Cu/Al coated ZnO nanorods could be a potential candidate for the UV light sensing. Furthermore, we can claim that Cu/Al coated ZnO nanorods is a promising alternative towards the development of earth abundant metal based stable and efficient nano-plasmonic devices.

Introduction

The field of plasmonics has rapidly grown over the past few years due to the interest in studying the behavior of light interacting with nano-scale material. Further, the surface plasmon (SP) arises due to the interaction of electromagnetic radiation with the conduction electrons presents at the metallic surface. Moreover, the resonance of SPs occurs when the frequency of electromagnetic radiation matches with the natural frequency of surface electrons. Such resonance frequencies or resonance wavelengths of SPs can be tuned by selection of metal nanostructures with different geometries, such as nanoparticles (NPs), nanorods (NRs), nanobelts (NBs) and nanowires (NWs) etc. Hence, metallic nanostructures could be promising for SPs as they can show strong enhancement in electromagnetic field intensities bounded to their surface [1]. Furthermore, it is very much interesting to find that the plasmonic nanostructures (NRs/NWs) can possess an exceptional capability to route, concentrate and manipulate the light at nanometer scale. Such unique properties defined them as a potential candidate for variety of optoelectronic applications, such as sensor, photodetector, solar cell, light-emitting diode, and laser diode etc [[2], [3], [4]].

Moreover, it is rigorously reported that the incorporation of plasmonic nanostructures in photodetectors overcomes the problems of light trapping and photon-generated carrier recombination in the optoelectronic devices [5]. Particularly by using metal nanoparticles, the maximum surface area to volume ration helps in scattering and absorption which are also depending on the wavelength of light and respective material properties. Therefore, surface plasmon resonance (SPR) involvement of metal nanoparticles in photodetectors shows great potential to be applied in more domains with higher performance and greater functionality [6]. In addition, semiconducting nanostructures present in photodetectors are also good in light gathering and luminescence which modifies their optical as well as electrical properties. Therefore, using both the elements (plasmonic metal and semiconductor) together could lead to a strong enhancement in the optical sensitivity of the system [7]. The optical properties of semiconductor nanomaterials are significantly improved due to the transfer of high energy electrons from metal surface to the surrounding semiconducting material [8]. Moreover, incorporating metal nanoparticles in semiconductor nanostructures is an efficient method to improve the quantum efficiency of different optoelectronic devices.

Besides, wide bandgap semiconductor-based ultraviolet photodetectors have attracted significant interest compared to the narrow bandgap semiconductors owing to their high quantum efficiency, enhanced photocurrent gain and large UV/visible rejection ratio [9]. Precisely, wide bandgap semiconductor nanostructures (nanorods/nanowires) are promising for improving the performance of UV photodetectors due to their high internal photoconductive gain resulting from surface-enhanced electron-hole separation efficiency [10]. Number of literature have reported that the ZnO is most widely studied n-type nanomaterial used for UV photodetection because of wide direct bandgap (3.37 eV), large exciton binding energy (60 meV), strong cohesive energy (1.89eV), high photoconductive gain and easy fabrication process [9].

However, pristine ZnO nanomaterial-based photodetectors frequently suffer from a major problem of slow response speed which arises due to the oxygen adsorption and desorption on the surface of ZnO nanostructures. Such problem eliminates their broader use in optoelectronic applications. Besides, most of the pristine ZnO nanostructures acquire a common problem of low emission enhancement of their photoluminescence (PL) spectra. They generally suffer from low near band edge (NBE) emission at the UV region and strong deep-level (DL) emission in the visible region. Such low NBE emission arises due to the presence of defect centers inside ZnO nanostructures during their synthesis process. Thus, ZnO nanostructures become unstable and low in carrier concentration [[11], [12], [13]]. It was studied from the previous reports that the passivation of DL emission can significantly improve the UV emission intensity. Several surface modification approaches have been attempted to improve the ZnO UV luminescence efficiency, such as thermal annealing, doping, surface plasmon, surface passivation, polymer covering and coating with metal nanoparticles [12,[14], [15], [16]]. However, the coating of metal NPs on ZnO nanostructures gain a considerable attention over past few years due to the SPR features of metal NPs. Incorporating various noble metal NPs such as Au, Ag and Pt. On ZnO nanostructures shows enhanced NBE (UV emission) emission and suppressed DL emission. Hence, combining ZnO with other materials to form heterojunctions is one of the most promising approach to solve the PL emission related problems and achieve high-speed UV photodetectors. Number of literature have reported that the combination of metal-ZnO nanostructures can significantly enhance the response of UV photodetection. Gogurla et al. have studied the SPR effect of Au NPs on ZnO nanosheet towards enhanced UV photodetection. They have observed ~80 times enhanced photo response of Au–ZnO nanocomposite in comparison to the pure ZnO nanostructures [17]. Lue et al. have studied a novel heterojunction UV photodetector based on Ag nanoparticles on ZnO NW arrays. They have observed that Ag–ZnO NW arrays showed enhanced light absorption, photocurrent, fast response and recovery time with on/off shift of UV light [10]. Similarly, Ning and coworkers have fabricated a self-powered ZnO nanofiber based UV photodetector where pure ZnO nanofibers and Ag-doped p-type ZnO nanofibers are processed to form p–n junction arrays for improving the performance in UV photodetection [18]. Moreover, Hu et al. have studied the performance of an UV photodetector made-up of p-Se/n-ZnO hybrid structure which showed faster response (fast speed in rise time and decay time) than pure ZnO nanostructures [19]. Tian and co-workers have investigated the enhanced performance of ZnO UV photodetector by incorporating Pt nanoparticles on ZnO thin film where they have observed enhanced responsivity of 0.47 A/W by sputtering Pt nanoparticles [9]. Moreover, Zhang et al. have synthesized millimeter-long ZnO/NiO heterojunction nanofiber arrays which have shown excellent ultraviolet (UV) selective and self-powered photodetection [20]. Similarly, Ouyang et al. have fabricated CdMoO4–ZnO composite film for UV photodetection and studied its performance enhancement with incorporation of Au nanoparticles [21]. Yao et al. have synthesized Sb-doped ZnO nanowires for nanoscale UV photodetector application [22]. Further, Echresh et al. have thermally evaporated p-type NiO thin film on M (Ag, Cd and Ni)-doped ZnO nanorods to fabricate high performance p-n heterojunction UV photodiodes [23]. Besides, in a different investigation Zheng and co-workers have fabricated p-Gr/AlN/n-ZnO heterojunction device for vacuum ultraviolet photodetection [24].

Hence, based on the reported literature we can predict that the SPR effect of noble/non-noble metals has strong impact towards enhancing the performance of UV photodetector. However, there are very limited research on SPR effect of earth abundant metal NPs towards improving the performance in ZnO based UV photodetectors. Therefore, our main objective is to investigate the SPR effect of earth abundant metal NPs, specially Cu and Al NPs on ZnO NRs for enhancing the performance in UV photodetection. The NPs were coated on ZnO NRs through pulsed laser deposition (PLD) technique which is a rather easy and quick process to cote the NPs on NRs. We can assume that the SPR effect of such earth abundant metal NPs could lead to improve the optical as well as electrical properties of ZnO nanorods. Therefore, Cu/Al coated ZnO NRs could be a highly competent for the low-cost photodetector applications.