Hybrid Ag/ZnO nanostructures for SERS detection of ammonium nitrate

Ag/ZnO composite nanostructures are produced by combined laser and ion implantation techniques. The ZnO layers are grown on SiO2 (001) and Al2O3 (r-cut) substrates by pulsed laser deposition (PLD) in vacuum and in oxygen ambient using a third-harmonic Nd:YAG laser. The ion implantation allows the introduction of Ag nanoparticles (NPs) in the surface of the ZnO matrix. These NPs are incorporated into the ZnO matrices to fabricate metal-semiconductor nanocomposites with the aim of manipulating their functionalities, exploiting the characteristics of both the matrix and the metal NPs. The composite samples are modified by laser annealing at 355 nm and 532 nm. The changes are investigated in the plasmon resonance absorption of the nanostructures before and after the annealing. The influence is explored of the different substrates used and the deposition conditions of ZnO growth on the properties of Ag/ZnO. The nanostructures obtained are efficient as SERS substrates for detection of ammonium nitrate under laser excitation at 633 nm. The SERS enhancement is attributed to the synergistic interactions between the plasmonic coupling among the surface embedded AgNPs and the enhanced charge transfer properties of the ZnO.


Introduction
The study of the collective effects arising in the interaction of an electromagnetic field with composite materials consisting of semiconductors functionalized by noble metal NPs is related to the development of efficient methods for fabrication of nanostructures with desirable optical parameters and their application in surface enhanced Raman spectroscopy (SERS) [1]. The latter is of interest for use in detection of substances with high social impact, such as nitrates, in food and water. Zinc oxide has received much attention because of its numerous technological applications [2]. The incorporation of noble metals as NPs in ZnO thin films is a very attractive way of enhancing their structural and optical properties [3,4]. The noble metals, in the form of nanoparticles, exhibit a selective surface plasmon resonance (SPR) band. The presence of plasmon-active nanoparticles in a matrix offers new optical properties for different plasmonic applications [5,6]. The nanocomposites consisting of metal  [7,8]. The interest in ion implantation as a method for synthesizing metal NPs in matrices is due to the use of composites in designing elements with unique optical properties [9]. Since the optical properties of metal nanoparticles are directly related to their size, one may control the optical performance of a composite as a whole by controllably varying the metal NP's size and size distribution by laser annealing [10]. These heterostructured nanocomposites with AgNPs arrays hold a great potential for SERS applications [11]. In this work, we describe a combined laser and ion implantation method for synthesis of hybrid Ag/ZnO nanostructures for SERS substrates.

Experimental
The ZnO layers are grown on SiO 2 (001) and Al 2 O 3 (r-cut) substrates by pulsed laser deposition (PLD) utilizing a third-harmonic Nd:YAG laser. The depositions are carried out in vacuum and in oxygen ambient at a pressure of 20 Pa. Shallow implantation of Ag + in the ZnO matrix is performed by an ILU-3 ion accelerator at room temperature with an energy of 30 keV, an irradiation dose of 10 17 ion/cm 2 and a current density of 5 μA/cm 2 .
The morphology of the samples and surface roughness are monitored before and after the ion implantation by a FastScan (Bruker) atomic force microscope (AFM). After the implantation, the samples are laser-annealed at two different wavelengths -355 nm and 532 nm, using 5 pulses at a laser fluence of 800 mJ/cm 2 and 750 mJ/cm 2 , respectively.
The transmission spectra are recorded in the spectral range 200 -800 nm by an HR 4000 UV-VIS spectrometer (Ocean Optics) to explore the plasmonic properties of Ag nanoparticles in ZnO environment before and after the laser modification by UV or VIS irradiation, respectively, 355 nm and 532 nm.
The SERS sensitivity of the nanostructures for ammonium nitrate (AN) is studied under excitation at 633 nm. The AN molecules are loaded by dropping a water solution on the surface of nanostructures; once the solution dries up, the Raman signal is collected. The samples are exposed to an AN concentration of 50 mg/l, this value being the drinking water exit standard at waterworks [12].

Results
The surface analyses of the samples' morphology performed by the AFM are shown in figure 1. The surface topography of the ZnO thin films on Al 2 O 3 and SiO 2 substrates is observed before and after the Ag + ion implantation. The fabricated ZnO layers demonstrate a relatively rough surface with columnar growth and an average size of the grains in the range 5 -60 nm depending on the substrate used ( figure 1 a and d). The films are crystalline and have a dense microstructure. The ion implantation results in the formation of AgNPs embedded on the surface as shown on figure 1 (b and e). The films deposited on sapphire substrates are characterized by the formation of smaller NPs and more uniform surface microstructure. The NPs mean size is about 13 nm with a narrow size distribution in the range 6 -20 nm. In comparison, the silver ion implantation in ZnO on a sapphire substrate leads to the formation of larger nanoparticles with a mean size of 35 nm and a size distribution in the range 10 -