Micro-strain governed photoluminescence emission intensity of sol-gel spin coated Eu doped ZnO thin films

The objective of the present study is to analyze the role of surface free energy and microstrain on optical properties of Eu- (1, 2, 3, 5 and 6 at.%) doped sol-gel spin coated ZnO film on glass substrates. X-ray diffraction results revealed the presence of (100) plane indicating highly oriented monocrystalline film. The crystallite size was in the range of 43 to 86 nm, and the microstrain in ZnO thin films decreased from 0.00079 to 0.00040 with Eu doping. UV–vis transmission indicated transparency of 80 to 95 percent with a dip at 377 nm for undoped ZnO, which was slightly displaced to 380 nm on doping with Eu. The reduction in band gap values calculated using Tauc's plot was in the range of 3.33 to 3.17 eV. The photoluminescence (PL) emission spectra demonstrated unaltered UV emission peak on doping with Eu, however, the reduction in PL intensity was governed by microstrain induced in ZnO thin film by Eu doping. PL intensity and microstrain both decreased with Eu doping up to 3 at.%, then increased for Eu 5 at.% and eventually decreased again for Eu 6 at.%. Field emission scanning electron microscopy results exhibited a variety of irregular multi-dimensional morphological characteristics. Rectangular, square, and spherical structures were observed in 1, 2, 5, and 6 at.% Eu doped ZnO thin film. In contrast, 3 at.% Eu doping revealed long chain linkage structures that stuck together. The water contact angle measurements established the hydrophilic nature of all the doped-films. The reduced surface free energy was implied by decreased contact angle for Eu doped ZnO thin films (1, 2, and 3 at.%). This result suggested that surface free energy can be tuned with the Eu-content in ZnO. Such synergistic optical and mechanical features in ZnO based materials can give opening to diverse applications ranging from brain inspired neural network to reduction of power consumption.