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Chandra V.K.,Chhatrapati Shivaji Institute of Technology | Chandra B.P.,Rani Durgavati University | Jha P.,Rani Durgavati University
Applied Physics Letters | Year: 2013

The luminescence induced by elastic deformation of solids, called the phenomenon of elastico-mechanoluminescence (EML), is observed in several materials. For applied pressure in the range of 17 MPa, certain crystals emit intense EML, which can be seen in day light with naked eye. In the present paper, we explore that, as the piezoelectric constant near the photo-generated electric dipoles formed by trapping of charge carriers in crystals is several times higher as compared to that at normal sites, the piezoelectrically induced detrapping of charge carriers and EML emission may take place for less value of the pressure applied onto the crystals. © 2013 AIP Publishing LLC.

Chandra V.K.,Chhatrapati Shivaji Institute of Technology | Chandra B.P.,Rani Durgavati University | Jha P.,Rani Durgavati University
Applied Physics Letters | Year: 2013

The long time dream of mechanoluminescence (ML) research to fabricate mechanoluminescence white light sources and mechanoluminescence displays seems to be turning into reality after the recent demonstration of highly bright and durable mechanoluminescent flexible composite films with a brightness of ≈120 cd/m2 and durability over ≈100 000 repeated mechanical stresses by using a combination of copper-doped zinc sulfide (ZnS:Cu) particles and polydimethylsiloxane. The present paper explores that self-recovery of mechanoluminescence of deforming piezoelectric semiconductors takes place by trapping of drifting charge carriers in the presence of piezoelectric field. This may be useful in enhancing the intensity and durability of ML devices. © 2013 AIP Publishing LLC.

Chandra V.K.,Chhatrapati Shivaji Institute of Technology | Chandra B.P.,Disha Institute of Management and Technology
Optical Materials | Year: 2011

Whereas the elastico mechanoluminescence (EML) of certain crystals increases linearly with the stress, nonlinearity occurs in the EML intensity versus stress plot of several crystals. The EML of crystals can be understood on the basis of piezoelectrically-induced detrapping model, whereby the localized piezoelectric field causes detrapping of electrons or holes and subsequently the capture of electrons in the excited states of activator ions, recombination of electrons in hole captured centres, recombination of holes in electron-captured centres or simply the electron-hole recombination gives rise to the light emission. Considering the piezoelectrically-induced detrapping model of EML expression is derived for the stress dependence of the EML intensity. It is shown that the crystals having uniform distribution of traps show linear relationship between the EML intensity and stress and the crystals having exponential distribution of traps show nonlinear relationship between the EML intensity and stress. The crystals having linear dependence of EML intensity on stress are suitable for the fabrication of EML-based stress sensors. The values of coefficient of deformation detrapping, relaxation time of the crosshead of the machine used to deform the samples and lifetime of the charge carriers in the shallow traps lying in the normal piezoelectric region of the crystals can be determined from the EML measurements. The values of the coefficient of deformation detrapping are 0.310, 0.018 and 0.021 MPa-1 for SrMgAl6O11:Eu, Sr2MgSi2O 7:Eu and SrCaMgSi2O7:Eu crystals, respectively. The coefficient of deformation detrapping is low for SrAl2O 4:Eu, SrAl2O4:Eu, Dy, SrBaMgSi 2O7:Eu and ZnS:Mn crystals and such crystals are suitable for EML-based stress sensors. A good agreement is found between the theoretical and experimental results. © 2011 Elsevier B.V. All rights reserved.

Chandra B.P.,Pandit Ravishankar Shukla University | Chandra V.K.,Chhatrapati Shivaji Institute of Technology | Jha P.,National Institute of Technology Raipur
Physica B: Condensed Matter | Year: 2015

Considering the detrapping of charge carriers due to reduction in trap-depth caused by piezoelectric field produced by applied pressure, an expression is derived for the detrapping rate of electrons. Then, an expression is obtained for the rate of generation of excited ions produced during capture of detrapped electrons by Eu3+ ions in persistent luminescent materials or by the energy released during electron-hole recombination in ZnS:Mn crystals. Finally, an expression is explored for the elastico-mechanoluminescence (EML) intensity, which is able to explain satisfactorily the characteristics of EML for the application of static pressure as well as for impact pressure. The total number of detrapped electrons and the total EML intensity are found to increase linearly with the electrostatic energy of the crystals in piezoelectric field. It is shown that the EML intensity should increase with the EML efficiency, number of crystallites (volume of sample), concentration of local piezoelectric regions in crystallites, piezoelectric constant of local piezoelectric regions, average length of the local piezoelectric regions, total number of electron traps, pressing rate, and applied pressure, and it should be higher for the materials having low value of threshold pressure and low value of trap-depth in unstressed condition. On the basis of the piezoelectrically-induced trap-depth reduction model of EML reported in the present investigation novel intense elastico mechanoluminescent materials having repetitive EML with undiminished intensity for successive loadings can be tailored which may find applications in sensing, imaging, lighting, colored displays, and other mechano-optical devices. © 2014 Elsevier B.V. All rights reserved.

Chandra B.P.,Rani Durgavati University | Chandra V.K.,Chhatrapati Shivaji Institute of Technology | Jha P.,Rani Durgavati University
Applied Physics Letters | Year: 2014

The concepts of piezoelectric field dependence of the detrapping probability and lifetime of charge carriers in traps are developed. The charge carriers in traps become unstable after a particular piezoelectric field whereby more number of charge carriers are detrapped with increasing piezoelectric field. The detrapped electrons tunnel to the energy level of the hole-captured activator ions lying adjacent to the conduction band or tunnel to conduction band. Subsequently, luminescence is induced by the electron-hole recombination. Using these concepts microscopic theory of elastico-mechanoluminescent smart materials is explored. Present study may be useful in tailoring intense elastico-mechanoluminescent materials needed for different applications. © 2014 AIP Publishing LLC.

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