Excitation wavelength dependent visible photoluminescence of CdO nanomorphotypes Paulose Thomas, K.E. Abraham n St. Berchmans College, Changanacherry 686101, Kerala, India article info Article history: Received 31 July 2014 Received in revised form 9 October 2014 Accepted 12 October 2014 Available online 22 October 2014 Keywords: CdO Nanomorphotypes Structural and molecular analysis Photoluminescence abstract We report the red-shifted photoluminescence (PL) band edge emission behavior as a function of subsequent increment in excitation wavelength on cadmium oxide (CdO) nanoflakes and nanoparalle- lepiped morphotypes prepared through the hydrothermal method. The unusual band edge PL emission lines in crystalline nanomaterials are explained as strong quantum size effect due to the size selective band gap widening in nanomaterials. This kind of excitation wavelength dependent PL emission behavior appears to be in violation of Kasha's rule of excitation wavelength independence of the emission spectrum. The surface morphology, particle size, molecular structure, chemical composition and chemical bonding of the samples are analyzed by SEM, TEM, XRD, EDS and FTIR spectroscopy. Direct optical band gap is calculated and found to be of 2.75 eV for nanoflakes and 2.52 eV for parallelepiped nanostructures. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The fabrication of different morphological nanostructured semiconductors has been a hype area of research due to their interesting properties [1–3]. CdO has been extensively studied in past decades and it has obtained many applications in different fields particularly in photoelectronics. Cadmium oxide is an n-type IIB–VIA, semiconductor with direct band gap of 2.5 eV and indirect band gap of 1.98 eV [1,4]. Therefore people have much interest to study its optical property particularly in photoluminescence (PL) analysis. There are numerous studies on the photoluminescence emission spectra of CdO nanostructures, but the change of the band edge emission spectral behavior with excitation wavelength is still insufficiently carried out. Kasha's rule suggests that material does not show the excitation dependent PL emission. But recently, the evolution of the PL emission line with excitation wavelength was observed in crystalline ZnO nanomaterials and suggested that every PL peak corresponds to an individual excitation wavelength [5]. Ellingson R.J. et al. observed evolution of excitation energy dependent efficiency of charge carrier relaxation and photolumi- nescence in colloidal InP quantum dots [6]. Micic et al. carried out a study on size-dependent PL spectroscopy of InP quantum dots and suggested that the emission and absorption features shift to higher energy with decreasing quantum dots size [7]. Similarly in the case of polymer-nanocomposite materials, fine excitation energy dependent photoluminescence property is observed. For example, Zhang et al. presented excitation-wavelength-dependent photoluminescence of a PMDI nanowires network and suggested that the luminescence peaks of PMDI nanowires red-shifted as the excitation wavelength increased [8]. Out of these facets metal oxide crystalline nanostructures have put on huge interest due to their capability to attain luminescence properties at various morphologies. It is well known that the particle sizes and morphologies of nanomaterials have immense influence on their optical properties. Accordingly, the controlled synthesis of nanostructured materials with novel morphologies has recently received much attention [9–10]. The morphology of the metal oxide nanomaterials has wide importance on their optical, electrical and magnetic properties. In the recent years many researchers have focused on synthesis of CdO in various nanomorphotypes, because of its promising applications in gas sensing, solar cells and flat panel displays [11–12]. CdO nanostructures synthesized in various attrac- tive morphotypes are nanwires [13], nanosheets, nanorods [1], nanobelts [14] and nanoparticles. The goal of present study is to investigate excitation dependent PL emission of crystalline CdO semiconductor nanomaterials in nanoflakes and nanoparallelepi- ped morphologies. In the present fabrication, cadmium hydroxide (Cd(OH) 2 ) nanostructure is a precursor product and the precursor nanostructure is calcined in air to convert it into CdO nanostructure. To the best of our knowledge this is the first ever made report of the CdO nanostructure with nanoflakes and nanoparallelepiped mor- phology. The present PL analysis reveals that the variation of band edge emission is due to the strong size selective band gap widening and confinement effect in nanomaterials. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2014.10.023 0022-2313/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 9447406915 (Mobile). E-mail address: abrahamke@gmail.com (K.E. Abraham). Journal of Luminescence 158 (2015) 422–427