Nanocrystalline zinc oxide (nc-ZnO) thin films were grown on p-type silicon substrate through spin coating by sol-gel process using different sol concentrations (10 wt.%, 15 wt.%, and 25 wt.%). These films were characterized by high resolution nondestructive X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS) attachment, and electron paramagnetic resonance (EPR) techniques to understand variations in structural, morphological, and oxygen vacancy with respect to sol concentration. The film surface morphology changes from nanowall to nanorods on increasing sol concentration. EPR spectra revealed the systematic variation from ferromagnetic to paramagnetic nature in these nc-ZnO films. The broad EPR resonance signal arising from the strong dipolar-dipolar interactions among impurity defects present in nc-ZnO film deposited from 10 wt.% sol has been observed and a single strong narrow resonance signal pertaining to oxygen vacancies is obtained in 25 wt.% sol derived nc-ZnO film. The concentrations of impurity defects and oxygen vacancies are evaluated from EPR spectra, necessary for efficient optoelectronic devices development.
ZnO is one of the most extensively explored nanomaterials of band gap energy ~3.37 eV with large exciton binding energy of 60 meV. The wide band gap of ZnO thin films encouraged their usage in the field of photonics and chemical sensors [
The chemical stability of nc-ZnO film has made it a suitable alternative candidate for conducting tin oxide and indium tin oxide films whose electrical and optical properties degrade in hydrogen plasma. Hence, the synthesis and characterization of nc-ZnO thin films have been an active area of research and development. Nanostructured ZnO thin films can be grown by different methods such as pulsed laser deposition, CVD, MOCVD, sputtering, electrochemical deposition, sol-gel, and spray pyrolysis [
The aim of the present studies was to deposit nc-ZnO thin films from different sol concentration and characterize their structural, morphological, optical, and paramagnetic defects/impurities variations by XRD, SEM, and EPR techniques. The sensitive and sophisticated EPR spectroscopy technique has been used for ascertaining the creation of paramagnetic nature defects in ZnO films and their variation with sol concentration.
nc-ZnO thin films were deposited on preoxidized single side polished (SSP)
As synthesized nc-ZnO films structural details were analyzed by Bruker AXS D8 Advance X-ray diffractometer using CuK
Zinc oxide formation by sol-gel process using zinc acetate dihydrate as precursor material consists of three steps, namely, hydrolysis, condensation, and polycondensation. In Step (1): hydrolysis reaction: zinc acetate dihydrate on dissolving in double distilled water forms partially hydrolyzed (basic zinc acetate) and mostly in ionic state (Zn2+ and acetate ion). The hydrolysis process is sensitive to the atmospheric relative humidity at ambient temperature which facilitates the formation of basic zinc acetate. In Step (2): this mixture solution on heating forms two molecules of Zn(OH)2. In Step (3): condensation/polycondensation reaction form chain of Zn-O with terminal OH groups. These three steps are represented by the following chemical equations: Hydrolysis reaction takes place on dissolution of zinc acetate in water: Ionized zinc acetate upon heating evaporates off zinc hydroxide and acetic acid while basic zinc acetate hydrolyzes to zinc hydroxide and acetate ions. These chemical reactions are given as Condensation/polycondensation reaction are given as
The polycondensation reaction is like a chain propagation reaction for
Zinc oxide crystallizes in hexagonal wurtzite structure with crystal lattice having alternate planes composed of tetrahedrally coordinated O2− and Zn2+ ions, bonded along the
XRD pattern of 10 wt.% sol concentration derived nc-ZnO thin film is presented in Figure
XRD patterns of 10% (inset) and 25% sol derived nc-ZnO thin film.
The dislocation density (
Structural parameters of nc-ZnO thin films derived from 10 wt.%, 15 wt.%, and 25 wt.% sol concentration.
nc-ZnO sol conc. | Plane |
|
FWHM ( |
2 |
|
|
|
---|---|---|---|---|---|---|---|
10 wt.% | 002 | 2.59402 | 0.06221 | 34.340 | 26.997 | 1.372 | 1.284 |
15 wt.% | 002 | 2.59582 | 0.04695 | 34.340 | 35.766 | 0.7817 | 0.9691 |
25 wt.% | 002 | 2.59633 | 0.03742 | 34.340 | 44.874 | 0.4966 | 0.7724 |
From Table
SEM micrographs of 10 wt.% and 25 wt.% sol derived ZnO thin film are presented in Figures
SEM micrograph: (a) 10 wt.% and (b) 25 wt.% sol derived nc-ZnO thin film and EDS of 25% sol derived nc-ZnO thin film.
The initial random orientation of the nanograins in the nanowall morphology has been attributed to the poor crystallization in low 10 wt.% sol concentration nc-ZnO film. When the sol concentration is increased to 25 wt.%, the dipole-dipole interaction between polar grains encourages the formation of nanorod/spindle shape morphology and orientation of nanograins along the
The atomic arrangement of the atoms present at surface of a thin film is studied by Scanning Tunneling Microscopy (STM) which detects the corrugations in the electron density on the surface of film. These corrugations are evolved from the positions of atoms on the surface. The three-dimensional profiles of different locations of the surface were generated to observe surface roughness, surface defects, and conformation of molecules and aggregates on the surface. As we know, that ZnO synthesis chemical reaction involves hydrolysis, condensation, and polycondensation of zinc hydroxide. During spin coating, the slip or dislocation in atomic arrangement of ZnO takes place more at the peripheries then in the center. The atomic arrangement in 100
STM image of 25 wt.% sol derived nc-ZnO thin film.
Electron paramagnetic resonance (EPR) spectroscopy is a very powerful sensitive technique used for the characterization of nature of magnetic impurities/defects and spin dynamics in materials that have paramagnetic defects [
EPR spectra of nc-ZnO thin films derived from (a) 10 wt.%, (b) 15 wt.%, and (c) 25 wt. % sol and (d) DPPH standard.
All the spectra obtained were analyzed using Lorentzian distribution function to obtain the values of various EPR parameters such as
EPR parameters evaluated from 10 wt.%, 15 wt.%, and 25 wt.% sol concentration of nc-ZnO thin films spectra.
Sample | Peak-to-peak linewidth |
|
Spin concentration |
Spin-spin relaxation |
---|---|---|---|---|
10 wt.% sol | 1265.52 | 2.0034 |
|
|
15 wt.% sol | 65.27 | 1.9673 |
|
|
25 wt.% sol | 159.42 | 1.9682 |
|
|
The spin concentration is calculated by the comparison method. In this method, the spins concentration in unknown sample is measured with respect to standard DPPH sample of known spin concentration which nullifies instrumental and environmental errors by recording EPR spectra of sample and standard reference sample under the same conditions. The spin concentration of paramagnetic centers present in unknown sample is calculated from the expression given as
The spin relaxation process deals with release of excited spin excessive energy to the surrounding spins and within lattice. EPR spectrum can be used to find the value of spin-spin relaxation time constant as represented the following equation:
As the sol concentration increases to 15 wt.%, the appearance of narrow signal superimposed on broad resonance signal confirms the coexistence of zinc vacancies related defects and oxygen vacancies present in film lattice. While in 25 wt.% sol derived nc-ZnO thin film, a narrow asymmetrical resonance signal of
In these investigations, nanocrystalline zinc oxide thin films were grown on silicon substrate by sol-gel method with different sol concentrations. XRD, SEM, EDS, STM, and EPR studies revealed the uniform, polycrystalline, hexagonal structure preferentially orientated along the
The authors declare that there is no conflict of interests regarding the publication of this paper.