Investigation on the Effects of the Formation of a Silver “Flower-Like Structure” on Graphene

Zakaria et al. Nanoscale Research Letters (2017) 12:50 DOI 10.1186/s11671-016-1793-y NANO EXPRESS Open Access Investigation on the Effects of the Formation of a Silver “Flower-Like Structure” on Graphene Rozalina Zakaria1* , Siti Fatimah Az Zahra Yusoff1, Kok Chung Law2, Chin Seong Lim2 and Harith Ahmad1 Abstract In this report, we experimentally investigate the formation of “flower-like silver structures” on graphene. Using an electrochemical deposition technique with deposition times of 2.5 and 5 min, agglomerations of silver nanoparticles (AgNPs) were deposited on the graphene surfaces, causing the formation of “flower-like structures” on the graphene substrate. Localized surface plasmon resonance (LSPR) was observed in the interaction between the structures and the graphene substrate. The morphology of the samples was observed using a field-emission scanning electron microscope (FESEM) and Raman spectroscopy. Thereafter, the potential of the flower-like Ag microstructures on graphene for use in Raman spectroscopic applications was examined. The signal showed a highest intensity value after a deposition time of 5 min, as portrayed by the intense local electromagnetic fields. For a better understanding, the CST Microwave Studio software, based on the finite element method (FEM), was applied to simulate the absorption characteristics of the structures on the graphene substrate. The absorption peak was redshifted due to the increment of the nanoparticle size. Keywords: Surface plasmon resonance, Silver nanoparticles, Electrochemical deposition technique, CST microwave studio Background Nanoplasmonics has been an intense subject in science and technology research fields that include the fabrication process and optical characterization of metal nanoparticles [1]. Surface plasmon resonance can be described as a quantum of electron charge density that oscillates at a metal-dielectric interface [2]. It has been reported that noble metal nanoparticles have unique electronic and optical properties. Particularly, plasmons on graphene are a new finding in nanoplasmonic applications, as they can confine a high amount of electromagnetic energy at subwavelength scales, thus leading to a strong surface plasmon resonance (SPR) at a metal-dielectric interface [3]. Moreover, because of the 2D characteristic of the collective excitations, the excitation of surface plasmons (SPs) in graphene becomes strongly restrained compared with those in normal noble metals [4]. Graphene thin films * Correspondence: 1 Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia Full list of author information is available at the end of the article decorated with random distributions of metallic nanoparticles have been shown to enhance optical and electronic properties, which has been studied by Reza et al. [5] by modeling the influence of graphene substrate with gold nanoparticles with distance on the graphene sheet. Graphene or 2D carbon sheets might be ideal nanoscale substrates to attain metal nanoparticle (NP) films [6]. Graphene itself has been recognized as an ideal optical material for optoelectronic applications replacing silicon and indium tin oxide because of the desirable properties offered, such as a good nonlinear effect, a low loss compared with usual metals [7], and a high carrier mobility. An effect of localized surface plasmon resonance (LSPR) exists due to the interaction between plasmonic nanoparticles, such as gold (Au) and silver (Ag), with the light wave on a surface of graphene enhancing the performance of nanoplasmonics, even though the use of Au and Ag is less efficient than copper (Cu) or nickel (Ni) [8]. A tunable surface plasmon on graphene can be achieved by varying the size, structure, and shape of the metal nanoparticles [9], thus resulting in a © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zakaria et al. Nanoscale Research Letters (2017) 12:50 variation of the wavelength of light detected. In this research, silver (Ag) nanoparticles were chosen due to their unique characteristics, such as high detection accuracy [10] and ability to provide a sharp SPR dip [11], which leads to a device with good sensitivity. In addition, Ag nanoparticles exhibit the strongest interaction between plasmonics and light and a greater scattering cross section compared with other noble metals [12]. In this study, the manner of LSPR of Ag nanoparticles coated on graphene surfaces with two different deposition times and structures was investigated as a preliminary approach. An electrochemical technique was applied to deposit Ag nanoparticles on a graphene substrate. However, the mechanism of the formation of flower-like silver nanostructures by the electrochemical technique has not been fully explored. Two samples with Ag nanoparticles were obtained at deposition times of 2.5 and 5 min to observe the formation of Ag nanoparticle structures. The morphology of the nanoparticles was then observed using a field-effect scanning electron microscope (FESEM). The existence of graphene was proven by Raman spectroscopy analysis with the addition of integrating metallic nanoparticles. A surface profiler was used to check the thickness of the Ag nanoparticles deposited on the graphene surface. In addition, the LSPR effect can be clarified by analyzing the absorption spectrum, whereby CST Microwave Studio (MMS) was used to simulate the light absorption characteristics of these two samples to support the experimental result. Methods Fabrication of Ag Nanoparticles The electrochemical deposition of Ag nanoparticles was conducted using a three-electrode electrochemical cell immersed in a silver ammonia ([Ag(NH3)2]OH) solution. First, 425 mg of AgNO3 was added to 50 mL distilled water to produce 50 mL of 50 mM AgNO3. The silver ammonia solution was obtained by mixing dropwise ammonia (1 wt%) with 10 mL of the AgNO3 solution (50 mM). The mixture was then stirred several times until the color of the solution changed to a light color, thus resulting in the formation of [Ag(NH3)2]OH with a concentration of 40 mM. There are two methods of electrochemical deposition—cyclic voltammetry (CV) and chronoamperometry (CA) electrodeposition—and these two processes were carried out in the prepared solutions using a potentiostat/galvanostat (Versastat 3 Applied Research Princeton, USA). The three-electrode system is described in Table 1 (below), and the schematic diagram of the system is shown in Fig. 1. Characterization of Ag Nanoparticles For the characterization of Ag nanoparticles on graphene, a Quanta 400F scanning (...truncated)