Detrapping Model Research Articles

Self-recoverable near-infrared mechanoluminescence from ZnS:Mn by controlling manganese clusterization

Near-infrared (NIR)-emitting mechanoluminescence (ML) materials are highly desirable since they are biological transparency and suitable for deep-imaging inside biological tissues. So far, NIR emission can be found in very limited number ML materials. Moreover, reported NIR ML materials suffer from limitations associated with the incompetence in self-reproducibility. Herein, we observe sustainable and self-recoverable NIR ML emission in pulsed laser deposition (PLD) method grown ZnS:Mn film, for the first time. ML emission in ZnS:Mn can be explained by the piezoelectricity-induced detrapping model. Broad ML emission covering both red and NIR ranges arises from the presence of (Mn)n clusters, which facilitates the energy transfer from excited Mn2+ ions to them. During the PLD growth, high laser fluence and deposition rate contribute to the formation of (Mn)n clusters, which was verified by electron paramagnetic resonance EPR and X-ray photoelectron spectroscopy XPS analysis. The device exhibits durable NIR ML over 105 repeated mechanical stresses, suggesting new possibilities aiming for constructing self-recoverable NIR ML materials.

Analysis and Compact Modeling of Fast Detrapping From Bandgap-Engineered Tunneling Oxide in 3-D NAND Flash Memories

We present the comprehensive analysis and the compact modeling methodology of fast electron detrapping from the bandgap engineered tunneling oxide (BE-TOX) in 3-D NAND flash memories. To this end, we develop a physics-based detrapping model considering the detailed electron dynamics in the BE-TOX and various emission paths of the detrapping mechanism. By the aid of the carrier density rates from the accurately calibrated model, we find that the major path of the charge loss can be direct tunneling (DT) or trap-to-band tunneling (TBT) depending on the temperature and the nitride layer position in the BE-TOX. In addition, we compare the model result and the fitted stretched exponential function. From those comparisons, we correlate several indicators (time constant and curve shape) of the retention characteristics with the major paths of the charge loss. Finally, our model facilitates the quantitative separation of the detrapping mechanism into different paths.

Largely enhanced elastico-mechanoluminescence of CaZnOS: Mn2+ by co-doping with Nd3+ ions

In this paper, a large enhancement of red elastico-mechanoluminescence (EML) intensity has been observed in piezoelectric semiconducting CaZnOS: Mn2+ phosphors via the co-doping of Nd3+ ions. The optimal EML intensity of CaZnOS: 1%Mn2+, 1.2%Nd3+ was about ten times than that of CaZnOS: 1%Mn2+. In addition, the relationship between the EML intensity and the applied load was almost linear and the phosphor exhibited recoverable performance under ultraviolet (UV) (254 nm) excitation, which is a potential candidate for pressure sensing sensor applications. The thermoluminescence (TL) results indicated the co-doping of Nd3+ ions not only increased the concentration of traps but also introduced two new types traps. Further investigations revealed that the shallow trap contributed the most to the long afterglow and two deep traps were mainly responsible for the EML performance. Based on the piezoelectricity-induced detrapping model, all the results suggested the enhanced EML intensity of CaZnOS: Mn2+ can attribute to the increased trap quantity and regulated trap concentration generated by the co-doping of Nd3+ ions.

The Modeling of Nitrogen Mass Transport in CoCr Alloys

Abstract The kinetics of plasma nitriding of CoCr alloy below temperatures of nitrides formation and mechanisms of nitrogen penetration are analyzed by proposed kinetic modeling in this article. Proposed nitrogen diffusion model is based on the trapping – detrapping (TD) model and developed taking into account the effect of the concentration dependent diffusivity of nitrogen, nitrogen adsorption on the surface of alloy and surface swelling process. The model indicates the influence of chromium atoms to nitrogen atoms diffusivity. The model consists of time and depth dependent diffusion, which is described by a partial differential equation, and it is solved by using Crank – Nicolson finite difference method. By fitting of experimental nitrogen depth profiles, it is shown that nitrogen diffusion coefficient varies with nitrogen concentration according to Einstein-Smoluchowski relation. Nitrogen depth profiles in plasma nitrided medical grade CoCr alloy (ISO 5831 – 12) at T = 400 °C for 1, 4 and 20 hours calculated on the basis of this model are in good agreement with experimental nitrogen profiles. Furthermore, the swelling process is showed and analyzed, derived the dependency of swelling rate on nitriding duration – the swelling rate is inversely proportional to the square root of nitriding duration. The obtained diffusion coefficient value and the swelling process rates satisfy the experimental data form Ref. The derived model explains physical processes during plasma nitriding and allows obtaining nitrogen depth profiles for any requisite nitriding duration.

Studies on the luminescence properties of europium doped strontium alumino-silicate phosphors by solid state reaction method

The Sr2Al2SiO7:Eu2+ and Sr2Al2SiO7:Eu3+ phosphors were prepared by the traditional high temperature solid state reaction method. The crystal structures of sintered phosphors were gehlenite type structure which belongs to the tetragonal crystallography with space group $$ {\rm P}\bar{42}_{1}{\rm m} $$ . Thermoluminescence properties of these phosphors were investigated and results were also compared. Under the ultra-violet excitation (345 nm), the Sr2Al2SiO7:Eu2+ phosphor would emit bluish-green (cyan) light, belonging to the broad emission band ascribed to the 4f 65d 1 → 4f 7 transition. The emission spectrum (λex = 395 nm) of Sr2Al2SiO7:Eu3+ phosphor were consisted of two emission peaks at 593 and 613 nm, which were attributed to 5D0 → 7F1 and 5D0 → 7F2 of Eu3+ ions. The strongest excited band at 395 nm can be assigned to 7F0 → 5L6 transition of Eu3+ ions due to the typical f–f transitions within Eu3+ of 4f 6 configuration. Commission International de I’Eclairage color coordinates of Sr2Al2SiO7:Eu2+ and Sr2Al2SiO7:Eu3+ phosphors were suitable as bluish-green and orange-red light emitting phosphors respectively. The mechanoluminescence (ML) intensity of Sr2Al2SiO7:Eu2+ and Sr2Al2SiO7:Eu3+ phosphors increase linearly with increasing impact velocity of the moving piston. Thus the present investigation indicates that the piezo-electrically induced de-trapping model is responsible to produce ML in sintered phosphors.

AVERT: An elaborate model for simulating variable retention time in DRAMs

In DRAMs, stored data on a capacitor tend to leak over time because of leakage current. To retain data, DRAMs require periodic refresh based on a profile of retention time. However, accurate DRAM refresh characterization is hindered by variable retention time (VRT) owing to random telegraph noise. In this paper, we propose AVERT, a device model and circuit simulation methodology for variable retention time in DRAMs. Based on the charge trapping and detrapping model, we generate a random telegraph signal in trap-induced gate leakage and trap-assisted gate-induced drain leakage to model random fluctuations in the retention time of DRAMs. With AVERT, we apply a stochastic device model of variable retention time to circuit simulation. Our results demonstrate the feasibility of simulating DRAM with random telegraph noise in leakage current and correspond closely to experimental results from prior publications. Based on observations from experimental data, we provide some insight into how to optimize the test time per cell and how to reduce the overall test time of a DRAM considering VRT.

Structural characterization and optical properties of dysprosium doped strontium calcium magnesium di-silicate phosphor by solid state reaction method

Dysprosium doped strontium calcium magnesium di-silicate phosphor namely: SrCaMgSi2O7:Dy3+ was prepared by the solid state reaction method. The crystal structure of the prepared phosphor was an akermanite type structure, which belongs to the tetragonal crystallography with space group P42‾1m. From field emission scanning electron microscopy (FESEM), agglomerations of particles were observed due to the high temperature synthesis process. The chemical composition of the sintered SrCaMgSi2O7:Dy3+ phosphor was confirmed by energy dispersive X-ray spectroscopy (EDX). Under the ultra-violet (UV) excitation, the characteristic emission of Dy3+ ions peaking at 478, 580 and 674nm, originating from the transitions of 4F9/2→6H15/2, 4F9/2→6H13/2 and 4F9/2→6H11/2 in the 4f9 configuration of Dy3+. Commission International de I’Eclairage (CIE) color coordinates of SrCaMgSi2O7:Dy3+ are suitable as white light emitting phosphor. Decay graph indicate that this phosphor contains fast decay and slow decay process. The mechanoluminescence (ML) intensity of SrCaMgSi2O7:Dy3+ phosphor increases linearly with increasing impact velocity of the moving piston, which suggests that this phosphor can be used as sensors to detect the stress of an object. Thus the present investigation indicates that the piezo-electrically induced de-trapping model is responsible to produce ML in SrCaMgSi2O7:Dy3+ phosphor. The possible mechanism of white light emitting long lasting phosphor is also investigated.

Structural characterization and luminescence properties of bluish-green-emitting SrCaMgSi2O7:Eu2+, Dy3+ phosphor by solid-state reaction method

An excellent long afterglow SrCaMgSi2O7:Eu2+, Dy3+ phosphor was prepared by the high-temperature solid-state reaction method. The phase structure of prepared phosphor was an akermanite-type structure, which belongs to the tetragonal crystallography with space group $$ {\text{P}}\overline{{{\text{42}}}} _{{\text{1}}} {\text{m}} $$ . Chemical composition of the sintered SrCaMgSi2O7:Eu2+, Dy3+ phosphor was confirmed by EDX. Under ultraviolet excitation, the SrCaMgSi2O7:Eu2+, Dy3+ phosphor would emit bluish-green (cyan) light, belonging to the broad emission band ascribed to the 4f65d1 → 4f7 transition. Investigation of afterglow properties show that SrCaMgSi2O7:Eu2+, Dy3+ phosphor held a fast and slow decay process. The peak of ML intensity increases linearly with increasing impact velocity of the moving piston. The CIE chromaticity diagram and ML spectra show a similar spectrum as PL spectra, which indicated that ML is emitted from the same emitting center of Eu2+ ions. Thus the present investigation indicates that the piezoelectrically induced de-trapping model is responsible to produce ML in prepared SrCaMgSi2O7:Eu2+, Dy3+ phosphor.

Mechanoluminescence of ZnS:Mn phosphors excited by hydrostatic pressure steps and pressure pulses

When a hydrostatic pressure step is applied rapidly on ZnS:Mn phosphor introduced into a pressure cell as oil suspension, initially the mechanoluminescence (ML) intensity increases linearly with time, attains a peak value for a particular time, and then it decreases with time (G. Alzetta, N. Minnaja, S. Santucci, Nuovo Cimento 23, 1962, 910). When a hydrostatic pressure pulse is applied onto ZnS:Mn phosphor, then two ML pulses of equal intensity are emitted; one during the application of pressure and the other during the release of pressure. In case of ZnS:Mn phosphor, at low hydrostatic pressure the energy produced during the electron–hole recombination excites the Mn2+ centres; however, at high hydrostatic pressure, the impact of accelerated electrons with the Mn2+ centres causes the light emission. Considering the piezoelectrically-induced detrapping model of ML at low pressure and the piezoelectrically-induced impact excitation model of ML at high pressure, expressions are derived for different characteristics of ML, in which a good agreement is found between the theoretical and experimental results. At low hydrostatic pressure in the range from 3MPa to 40MPa, piezoelectrically-induced detrapping model of ML becomes applicable in ZnS:Mn phosphors; while at high hydrostatic pressure beyond 40MPa, the piezoelectrically-induced impact excitation model of ML becomes applicable. The ML induced by hydrostatic pressure can be used for sensing both the magnitude and rise time of applied hydrostatic pressure.

Novel elastico-mechanoluminescence materials CaZnOS:Mn2+ and CaZr(PO4)2:Eu2+

This paper highlights our work on the recent progress in novel elastico-mechanoluminescence (EML) materials CaZnOS : Mn 2+ and CaZr ( PO 4)2: Eu 2+, which can simultaneously "feel" (sense) and "see" (image) the applied mechanical stress as an intense and proportional luminescence. CaZnOS : Mn 2+ can sense various types of mechanical stress, including ultrasonic vibration, impact, friction and compression because of the large piezoelectric coefficient. CaZr ( PO 4)2: Eu 2+ with multiple trap levels shows a broad measurement range for dynamic load. Both of CaZnOS : Mn 2+ and CaZr ( PO 4)2: Eu 2+ belong to the defect-controlled type piezoelectric phosphors, and their EML mechanisms could be explained using the piezoelectrically induced carrier de-trapping model.

Pulse-induced mechanoluminescence of ultraviolet-irradiated SrAl2O4:Eu,Dy phosphors

The SrAl2O4:Eu,Dy phosphors prepared by solid state reaction technique in a reduced atmosphere of 95% Ar+5% H2 exhibit very intense mechanoluminescence (ML) which can be seen in daylight with naked eye. When the phosphors are deformed by the impact of a low-power electric hammer, initially the ML intensity increases with time, attains a maximum value and then decreases with time. After the threshold pressure, the peak of ML intensity Im and the total ML intensity IT increase with the increasing value of the impact pressure. For the ML excited by the pressure pulse of short duration, two decay times of ML are observed; however, for the ML excited by the pressure pulse of long duration, only one decay time is observed. The ML intensity decreases with successive applications of pressure on SrAl2O4:Eu,Dy phosphors. For the low applied pressure in the range below the limit of elasticity recovery of ML intensity takes place when the sample is exposed to ultraviolet (UV) light. This fact indicates that the vacant traps produced during the application of pressure pulses get filled during the exposure of the sample to UV light. The ML in the elastic region of SrAl2O4:Eu,Dy phosphors can be understood on the basis of the piezoelectrically induced detrapping model. The non-irradiated SrAl2O4:Eu2+,Dy3+ phosphors exhibit ML during the fracture of the compact mass of phosphors whose ML intensity is less when compared to that of the UV-irradiated compact masses. The ML induced by pressure pulses may be useful for determining the magnitude and rise time of unknown pressure pulses and to determine the lifetime of charge carriers in shallow traps.

Elastico-Mechanoluminescence of Thermoluminescent Crystals

Elastico-mechanoluminescence (EML) is a type of luminescence induced by elastic deformation of solids. The present paper reports the elastic-ML of thermoluminescent crystals such as X-or γ-irradiated alkali halide crystals, ZnS:Mn, and ultraviolet irradiated persistent luminescent crystals. Generally, all the elastico-mechanoluminescent crystals are thermoluminescent, but all the thermoluminescent crystals are not the mechanoluminescent. The elastico-mechanoluminescence spectra of crystals are similar to their thermoluminescence spectra. Both the elastico-mechanoluminescence and thermoluminescence arise due to the de-trapping of charge carriers. As elastico-ML of persistent luminescent crystals depends on both the density of filled traps and piezoelectric field, the intense thermoluminescent crystals may not be the intense mechanoluminescent crystals. When a sample of X-or γ-irradiated alkali halide crystal, UV-irradiated persistent luminescent microcrystals mixed in epoxy resin, or a film of ZnS:Mn nanoparticles is deformed in the elastic region by the pressure rising at fixed pressing rate for a particular time, or by a pressure of triangular form, or by a pressure pulse, then after a threshold pressure, initially the EML intensity increases with time, attains a maximum value and later on it decreases with time. In the first case, the fast decay time of EML is related to the time-constant for stopping the moving crosshead of the testing machine; in the second case, generally the fast decay does not appear; and in the third case, the fast decay time is equal to the rise time of the pressure pulse. However, in all the cases, the slow decay time is related to the lifetime of re-trapped charge carriers in the shallow traps lying in the region where the piezoelectric field is negligible. When the sample is deformed by the pressure rising at fixed pressing rate for a particular time, or pressure of triangular form, then the ML appears after a threshold pressure and the transient EML intensity increases linearly with the applied pressure; however, the total EML intensity increases quadratically with the applied pressure. The EML intensity of persistent luminescent crystals decreases with increasing number of pressings. However, when these crystals are exposed to UV light, then the recovery of EML intensity takes place. The mechanical interaction between the bending segment of dislocations and filled electron traps is able to explain the elastico-ML of X-or γ-irradiated alkali halide crystals. However, the piezoelectrically-induced de-trapping model is suitable for explaining the ML of persistent luminescent crystals and ZnS:Mn. The investigation of elastico-ML may be helpful in understanding the thermoluminescence and the investigation of thermoluminescence may be helpful in understanding elastico-ML. Furthermore, similar to the thermoluminescence, the mechanoluminescence may also find application in radiation dosimetry. Expressions are derived for the elastico-ML of thermoluminescent crystals, in which a good agreement is found between the experimental and theoretical results. Finally, the application of the elasticoML of thermoluminescent crystals in light sources, displays, imaging devices, sensing devices, radiation dosimetry and in non-destructive testing of materials are discussed.Contents of Paper

Impulsive excitation of mechanoluminescence in SrAl2O4:Eu, Dy phosphors prepared by solid state reaction technique in reduction atmosphere

The present paper reports the impulsive excitation of mechanoluminescence (ML) in SrAl2O4:Eu, Dy phosphor prepared by solid state reaction technique in a reduced atmosphere of 95% Ar+5% H2. The phosphors prepared exhibit such as intense ML that it can be seen in day light with naked eye. When a sample consisting of compact mass of the phosphor is deformed impulsively by the impact of a moving piston, then initially the mechanoluminescence intensity increases linearly with time, attains a peak value Im at a particular time tm, and later on it decreases with time, initially at a fast rate and then at a slow rate. The peak ML intensity Im and total ML intensity IT increase quardratically with the applied pressure and impact velocity. The ML spectra of SrAl2O4:Eu, Dy are found to be similar to the photoluminescence spectra. This result indicates that the ML is emitted from the Eu2+ ions as photoluminescence does. The ML intensity decreases with successive impact of the load onto the phosphors, whereby the diminished ML intensity can be recovered approximately by UV-irradiation. retain-->In the impulsive excitation, the complete recovery of ML intensity does not take place because the probability of radiative transition of luminescence centres formed near the newly created surfaces may decrease due the formation of surface traps. The values of tm, fast decay time and slow decay time do not change significantly with the impact velocity vo. The piezoelectrically-induced detrapping model is able to explain the characteristics of ML. According to this model, the local piezoelectric field near the activators causes the detrapping of electrons trapped in the negative oxygen vacancy and subsequently the detrapped electrons moving in the conduction band are captured by the Eu3+ ion, whereby the excited Eu2+ ions are produced and the subsequent de-excitation of excited Eu2+ ions gives rise to the light emission. It seems that, in addition to the change in crystal structure, in UV-irradiated SrAl2O4:Eu, Dy crystals, some localized electric dipoles present in the vicinity of Eu2+ ions may also enhance the piezoelectric constants of the crystals. The day light visible mechanoluminescence in SrAl2O4:Eu, Dy phosphors may be used for designing the impact stress and impact velocity sensors. Furthermore, the present investigation may be useful for determining the rise time of unknown pressure pulses and for determining the lifetime of charge carriers in shallow traps. The intense ML pulses may also be used in some specific photocatalytic reaction. The present study may also be helpful in preparing the ML-based light source by increasing the trap-depth of shallow traps. If the ML emission in certain materials will be in the ultraviolet range, then the ML pulses of short duration may be used to excite the phosphorescence and determining the decay time.

Modelling of time-dependent dielectric barrier breakdown mechanisms in MgO-based magnetic tunnel junctions

An investigation of barrier endurance till electrical breakdown in MgO-based magnetic tunnel junctions (MTJs) is presented. Samples were tested under pulsed electrical stress. By studying the effect of delay between successive pulses, an optimum endurance of MTJs is observed for an intermediate value of delay between pulses corresponding to an optimum trade-off between the average density of charge trapped in the barrier and the amplitude of its time-modulation at each voltage pulse. A charge trapping–detrapping model was developed which shows good coherence with experimental results. The influence of the delay between pulses on the trapped charge density in the tunnel barrier and on its time-modulation is discussed. The average density of trapped charges and its time-modulation are, respectively, responsible for a static and dynamic stress within the tunnel barrier, both leading to breakdown. The probability of breakdown of the MTJ for different applied pulse conditions has been evaluated. The expected endurance of the MTJs was deduced depending on the characteristics of the electrical stress in terms of delay, and unipolarity versus bipolarity. The study emphasizes the role of electron trapping/detrapping mechanisms on the tunnel barrier reliability. It also shows that extremely long endurance could be obtained in MTJs by reducing the density of electron trapping sites in the tunnel barrier.

Characterization of environment-dependent hysteresis in indium gallium zinc oxide thin film transistors

This study investigates environmentally dependent electrical performance as a function of hysteresis phenomena for amorphous indium–gallium–zinc-oxide (a-IGZO) thin film transistors (TFTs). Because ambient gas has a great influence on the electrical characteristics of zinc oxide-based TFTs during electrical measurements, the hysteresis for current–voltage and capacitance–voltage measurements are employed in different ambient gases (ambient air, vacuum, oxygen ambient and moisture ambient) to study this issue. Although the hysteresis phenomenon has been explained by the charge trapping and de-trapping model, in order to obtain stable electrical characteristics it is necessary to clarify the original principal mechanism leading to threshold voltage instability under gate bias operations in different environments. Also, the relationship between de-trapping rate and electrical hysteresis is investigated by prolonging the integration time of the semiconductor parameter analyzer. This study may be of importance in explaining the dynamic relationship between the TFT electrical characteristics and ambient gas, as well as providing a better understanding of the environment-dependent hysteresis phenomenon of the IGZO TFTs.

A space charge trapping model and its parameters in polymeric material

In this paper, characteristics of charge trapping and detrapping in low density polyethylene under dc electric field are investigated using the pulsed electroacoustic technique. It is found that the charge decay shows very different features for the samples with different periods of applied electric field. A simple trapping and detrapping model based on two trapping levels is proposed to qualitatively explain the observation. At the same time, numerical simulation based on the above model is carried out to extract parameters (trap depths and concentration) related to the material. It is found that the space charge decaying in the first few hundred seconds, corresponding to the fast changing part of the slope, is trapped in a shallow trap with a depth in a range between 0.77 and 0.81 eV, and the trapped charge density reaches (1.1681.553) 1019 m-3 in the sample volume measured. At the same time, the space charge that decays at longer time, corresponding to the slower part of the slope, is trapped in a deep trap with a depth in a range of 0.96 and 1.01 eV, and the trapped charge density is (1.1944.615) 1018 m-3. The trap depths and charge densities of both shallow and deep traps may increase with ageing, and the parameters of two energy wells can be used as an indication of the material aging.

Mechanoluminescence of Nanoparticles

Certain nanoparticles exhibit intense light emission during their elastic deformation, called the phenomenon of elastico mechanoluminescence (EML).When ZnS:Mn and SrAl2O4:Eu nanoparticles are deformed in the elastic region by the stress initially increasing with time and then attaining a fixed value or by the stress initially increasing and then decreasing with time or by the impact stress, then after a threshold pressure, initially the EML intensity increases with time, attains a maximum value and later on it decreases with time. For the fixed pressure or slowly decreasing pressure, the slow decay time of EML is equal to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals.In these cases, sometimes, fast decay of EML also appears and it is related to the machine -constant, that is, the decay time of strain- rate after the power to the cross-head of the machine is switched off. For the fast-deformation caused by impact stress, the fast decay time of EML is controlled by the rise time of impact stress, and the slow decay of EML gives the lifetime of retrapped electrons in the shallow traps. For ZnS:Mn and SrAl2O4:Eu nanoparticles, the total EML intensity increases quadratically with the applied pressure. The diminished EML intensity of SrAl2O4:Eu nanoparticles caused by number of pressings can be recovered by exposing the sample to UV-radiation. The tribo ML of ZnS:Mn nanoparticles increases with increasing value of the load placed on the rotating rod inducing tribo ML. The EML spectra of ZnS:Mn and SrAl2O4:Eu nanoparticles are similar to their photoluminescence and electrolumi- nescence spectra. The piezoelectrically-induced detrapping model is found to be suitable for the EML of nanoparticles and the expressions derived on the basis of this model are able to explain successfully the characteristics of the EML of nanoparticles. The EML of nanoparticles has been found useful in stress sensor and an EML-based safety-management monitoring system has been developed. The EML of nanoparticles can be used for the visualizations of stress distribution in solids, stress field near crack-tip, quasidynamic crack-propagation in solids and internal defect in a pipe. The EML intensity of nanoparticles is so intense that it is able to drive a solar cell system and it can be used as a light-source for specific applications. Furthermore, several parameters of nanoparticles can be determined from their EML measurements.

Dynamics of the mechanoluminescence induced by elastic deformation of persistent luminescent crystals

When a composite of suitable dimension formed by mixing the microcrystalline or nanocrystalline persistent luminescent materials in epoxy resin is deformed at a fixed pressing rate, then the elastico mechanoluminescence (EML) emission takes place after a threshold pressure, in which the EML intensity increases linearly with the applied pressure. When the applied pressure is kept constant or decreased linearly, then the EML intensity decreases with time, in which depending on the prevailing condition, the EML intensity initially decreases at a fast rate and then at a slow rate or sometimes it decreases exponentially having only one decay time. When a small ball is dropped from a low height onto the film of a persistent luminescent material, then initially the EML intensity increases with time, attains a peak value and then it decreases initially at a fast rate and later on at a slow rate. In this case, both the peak EML intensity and the total EML intensity increase linearly with the height through which the ball is dropped onto the film. Considering the piezoelectrically induced detrapping model based on successive detrapping of exponentially distributed traps a theoretical approach is made to the dynamics of light emission induced by elastic deformation of persistent luminescent crystals and thin films. It is shown that the EML intensity depends on several parameters such as pressure, pressing rate or strain rate, temperature, density of filled electron traps, piezoelectric constant near defect centers, etc. Both, in the case of slow deformation and impact stress, the fast decay time is related to the time-constant for the decrease of pressing rate of the samples and the slow decay time of EML is related to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals. Both, the EML produced during the release of pressure and the EML produced during the successive applications of pressure take place due to the detrapping of retrapped electrons in the vacant electron traps near activator ions, in which retrapping is caused by the thermally released electrons from the filled shallow traps lying in the normal piezoelectric region of the crystals, which get filled during the detrapping of stable traps at the time of increase of pressure. On the basis of the proposed model, the dependence of EML intensity on different parameters, dynamics of EML and physical concepts of the threshold pressure, characteristic piezoelectric field for detrapping, coefficient of deformation detrapping, nonlinear increase of the EML intensity of some crystals at high pressure and higher EML intensity in the crystals having higher coefficient of deformation detrapping can be satisfactorily understood. A good agreement is found between the theoretical and experimental results. It is shown that the present study may be helpful in tailoring the intense persistent elastico mechanoluminescent materials having long lasting time.

Real-time mechanoluminescence sensing of the amplitude and duration of impact stress

The present paper reports the real-time sensing of the amplitude and duration of impact stress using mechanoluminescence (ML) of the films such as ZnS:Mn and SrAl 2O 4:Eu. After the impact of a small ball from a low height onto the film, initially the elastico mechanoluminescence (EML) intensity increases with time, attains a peak value and then it decreases with time, initially at a fast rate and later on at a slow rate. The fast decay time of the EML intensity is related to the rate constant for the rise of impact stress and the slow decay time of EML is equal to the lifetime of electrons in the shallow traps lying in the normal piezoelectric region of the crystals, which get filled during the detrapping of thermally stable traps at the time of the increase of pressure. Both the peaks of EML intensity and total EML intensity increase linearly with the height through which the ball is dropped onto the films. The EML spectra are similar to the corresponding photoluminescence and electroluminescence spectra. On the basis of the localized piezoelectrically induced electron detrapping model, expressions are derived for different parameters of the impact stress-induced EML of the films, whereby a good agreement is found between the experimental and theoretical results. As the EML intensity depends on the impact stress, the impact stress can be sensed by measuring the EML intensity. Furthermore, the duration of stress is related to the time t m corresponding to the peak of the EML intensity versus time curve; hence, the pulse duration of the impact stress can be monitored by measuring the value of time t m .

Suitable materials for elastico mechanoluminescence-based stress sensors

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 SrMgAl 6O 11:Eu, Sr 2MgSi 2O 7:Eu and SrCaMgSi 2O 7:Eu crystals, respectively. The coefficient of deformation detrapping is low for SrAl 2O 4:Eu, SrAl 2O 4:Eu, Dy, SrBaMgSi 2O 7: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.