Applied Surface Science 258 (2012) 9181–9185 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Enhancement of ZnO photoluminescence by laser nanostructuring of Ag underlayer M.E. Koleva ∗ , A.Og. Dikovska, N.N. Nedyalkov, P.A. Atanasov, I.A. Bliznakova Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee blvd., Sofia 1784, Bulgaria a r t i c l e i n f o Article history: Available online 3 February 2012 Keywords: PLD Laser nanostructuring Ag nanoparticles nanocomposites ZnO photoluminescence Surface plasmon resonance a b s t r a c t Results are presented on the synthesis and characterization of nanocomposite Ag/ZnO heterostructures on fused quartz substrates. The experiments were performed by pulsed laser deposition (PLD) using a third harmonic Nd:YAG laser in a vacuum chamber. The ablation of a rotated Ag target was followed by laser modification of the deposited layer and subsequent deposition of ZnO. The process factors, such as the silver film thickness and the post-deposition annealing procedures, were varied in view of controlling the diameter and size distribution of the Ag nanoparticles. The annealing regimes were modified by varying the laser irradiation energy on the Ag layers. The effect on the Ag/ZnO properties is discussed. It is demonstrated that the photoluminescence intensity of ZnO nanolayers is significantly enhanced by plasmon absorption in the Ag nanoparticles and a shift to the lower wavelengths. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The wide bandgap and large exciton binding energy of zinc oxide (ZnO) have attracted enormous interest in view of poten- tial applications in electronic and opto-electronic devices [1,2]. Besides the intensive studies of single-component ZnO nanostruc- tures, efforts have also been devoted to the fabrication of hybrids consisting of ZnO and a noble metal to achieve improved proper- ties and extended applications [3,4]. In general, the PL spectrum of ZnO consists of two bands, a near-band-edge excitonic UV emis- sion and a defect-related deep-level emission in the visible range [5]. The two-carrier recombination routes compete with each other during the luminescence process [6]. The existence of ZnO PL band-edge enhancement due to both charge-transfer mechanisms and local-field mechanisms creates a fundamental problem in studying exciton–plasmon interactions using rough metal films as the source of plasmonic interactions [7]. The mechanisms for coupling of ZnO excitons to silver plas- mons with the purpose of spectral control and amplification of the band-edge photoluminescence (PL) have attracted particular atten- tion in recent years [8–10]. The energetics and dynamics of this interaction was described by Haglund et al. [7] based on multilay- ered heterostructures that comprise MgO-doped and undoped ZnO and Ag. The PL efficiency enhancement is attributed to both local- ized surface plasmon resonances (LSPRs) and propagating surface plasmon polaritons (SPPs) [11]. Plasmon–exciton interactions can ∗ Corresponding author. Tel.: +3592 9795911. E-mail address: mihaela ek@yahoo.com (M.E. Koleva). amplify the band-edge PL by a factor of 10 in Ag–ZnO bilayers [12], and by as much as 20 in Au–ZnO nanoparticle composites [13]. Bearing in mind the different coupling mechanisms in bilayers and nanoparticle composites, our efforts were focused on investigating and improving the emitting properties of ZnO films depending on the morphology and optical properties of the Ag nanoparticles. 2. Experimental The Ag films were deposited in a vacuum chamber at a base pressure of 4 × 10 -2 Pa by using the third harmonic of a Nd:YAG laser (Lotis LS-2147;  = 355 nm, pulse duration 18 ns) operating at a repetition rate of 10 Hz. The laser beam with a fluence of 1.1 J/cm 2 was focused onto the rotating Ag target for uniform abla- tion. The deposition rate was 60 nm/min with the deposited films having thicknesses of 120 and 180 nm. The proper choice of the refractive index of the substrates and emitting layers is decisive for the plasmon coupled light emission enhancement [12]. The posi- tive effect of the quartz substrate on the ZnO photoluminescence efficiency has been proved. In our experiments, the as-deposited films were laser irradiated in the fluence range of 0.60–1 mJ/cm 2 by a fixed number of laser pulses N p = 10 to achieve decomposition into nanoparticles. The nanostructured surfaces were investigated by scanning electron microscopy (dual beam system SEM/FIB, Lyra\Tescan) and the mean size of Ag nanoparticles was evaluated. The transmission spectra of the Ag nanoparticles were analyzed using a VIS OTS spectrometer (Ocean Optics Inc.) in the range 360–780 nm. Using the same deposition equipment, the nanopar- ticles were covered by a ZnO film by PLD at room temperature and oxygen pressure of 3 Pa for 10 min. The laser fluence was 1.7 J/cm 2 . 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.052