Single step synthesis and optical limiting properties of Ni–Ag and Fe–Ag bimetallic nanoparticles Kishore Sridharan a,c , Tamio Endo b , Sang-Geun Cho a , Jongryoul Kim a , Tae Joo Park a,⇑ , Reji Philip c,⇑ a Department of Materials Engineering, Hanyang University, Ansan 426 791, Republic of Korea b Department of Electrical and Electronics Engineering, Mie University, Tsu, Mie 514 8507, Japan c Light and Matter Physics Group, Raman Research Institute, C.V. Raman Avenue, Sadhashivanagar, Bangalore 560 080, India article info Article history: Received 22 August 2012 Received in revised form 23 October 2012 Accepted 24 October 2012 Available online 21 December 2012 Keywords: Bimetallic alloy nanoparticles Surface plasmon resonance Optical limiting Nonlinear absorption Ultrafast laser excitation abstract Magnetic nanoparticles have several applications in biology and medicine, and recently, their use for optical applications is gaining substantial attention. In this paper we report a single step solution based synthesis of Ni–Ag and Fe–Ag bimetallic nanoparticles using hydrazine hydrate as the reducing agent. Structural, plasmonic, and nonlinear optical properties of the prepared nanoparticles are investigated using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), UV–Vis absorption spectroscopy and Z-scan. UV–Vis absorption studies reveal surface plasmon resonance (SPR) absorption at 400 nm which exhibits a small red shift with particle size due to scattering. XRD, EDS, HRTEM and SPR absorption confirm the presence of Ni–Ag and Fe–Ag nanoparticles. Nonlinear opti- cal studies are carried out using the open aperture Z-scan technique employing 5 ns as well as 100 fs laser pulses. The nonlinearity is found to have contributions from absorption saturation, two-photon absorp- tion, excited state absorption and induced thermal scattering of light. The effective nonlinear absorption in Ni–Ag and Fe–Ag nanoparticles is found to be higher than that of pristine Ag nanoparticles. The poten- tial of these materials for optical limiting and photonic applications is discussed. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Metal particles in the nanometer size regime show size-depen- dent properties different from those of bulk metals, which arouse substantial interest in their investigations. Noble metal nanoparti- cles such as copper, gold and silver have a strong surface plasmon resonance (SPR) in the visible spectral region caused by the coher- ent motion of conduction band electrons, whereas most other tran- sition metals show only a broad and poorly resolved absorption band in the ultraviolet region [1,2]. This difference is attributed to the strong coupling between the plasmon absorption and the interband excitation [3]. Properties of metals can be significantly modified by using a mixture of elements to generate inter-metallic compounds. Colloidal nanoparticles, either alloyed or core-shelled, have attracted significant attention due to the new properties that emerge from the combination of different metals in the nanoscale and there is a significant enhancement in the physical and chemi- cal properties due to synergistic effects [4,5]. For example, Nishida et al. [6] studied the electro-optical properties of liquid crystal molecule capped Ag–Pd bimetallic nanoparticles. He found out that the Ag–Pd nanoparticles were stable than liquid crystal mole- cule capped Ag nanoparticles, and surface composition of the bimetallic nanoparticles had an effect on the properties of the con- structed liquid crystal display (LCD). The surface plasmon reso- nance property of Au–Ag bimetallic nanoparticles has been employed in surface enhanced raman spectroscopy [7,8] and in fi- ber optic sensors [9]. In the field of catalysis a variety of bimetallic nanoparticles such as CuPd, CuRh, AuPd, AuRh, PtRh, PdRh and AuPt supported on ceria have been reported to show enhanced cat- alytic activity for the oxidation of CO [10]. Moreover it has been found out that bimetallic nanoparticles can exhibit better catalytic performance than pristine Pt nanoparticles because of the bifunc- tional reaction mechanism of bimetallic alloy electrocatalysts [11]. Similarly the magnetic properties of bimetallic FePt nanopar- ticles [12,13] have been well accounted and its use in high density magnetic recording is well known. Diversity in synthesis and the resulting tunability of the physi- cal and chemical properties are the major reasons why metal nanoclusters are of special interest for applications in plasmonics and nonlinear optics (NLO). Phase diagram of Nickel and silver [14] shows that they are thermally immiscible and so they do not form an alloy in the macroscale. In transition metal nanoparti- cles, a strong extinction coefficient originates from the interband and intraband transitions that hinders dipolar plasmon excitation. Nevertheless while associating noble metals to transition metals, the contribution of conduction electrons leads to the improvement in the localized surface plasmon resonance (LSPR), which was 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.10.053 ⇑ Corresponding authors. E-mail addresses: tjp@hanyang.ac.kr (T.J. Park), reji@rri.res.in (R. Philip). Optical Materials 35 (2013) 860–867 Contents lists available at SciVerse ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat